James & Wells Intellectual Property Ref: 234270AU3/53 DRILLING APPARATUS STATEMENT OF CORRESPONDING APPLICATIONS This application is based on the complete specification filed in relation to Australian Patent Application No. 2008904823, the entire contents of which are incorporated 5 herein by reference. TECHNICAL FIELD This invention relates to a drilling apparatus. More particularly, this invention relates to a hydraulic "down-the-hole" (DTH) percussion drilling apparatus for drilling holes in a terrain. 10 BACKGROUND ART The service life of hydraulic DTH systems used in situations requiring the addition and removal of rods during drilling is adversely affected by contamination introduced to the oil during rod changes. Controlling and/or removing contamination is a key technical hurdle to be overcome in the pursuit of system 15 reliability and longevity and thus the reduction in overall drilling cost- Controlling contamination requires that the surfaces that are exposed to oil during operation of the system (i.e. the pipe connection valves) are isolated from the surrounding potentially dusty environment. There are several options for contamination control which can be built into drilling 20 rig design. However the DTH system itself must also contain measures for contamination control and/or removal which provide effective protection regardless of the design of the drilling rig to which it is fitted. It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice. 1 James & Wells Intellectual Property Reft 234270AU3/53 Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any 5 reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common 10 general knowledge in the art, in Australia or in any other country. It is acknowledged that the term 'comprising' may. under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprising' shall have an inclusive meaning - Le. that it will be taken to mean an inclusion of not only the 15 listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process. DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a drilling 20 apparatus comprising: a at least one drill rod comprising: o a connection valve for connection to a connection valve of a like drill rod or a connection valve of a hydraulically powered hammer; wherein 2 James & Wells Intellectual Property Ref: 234270AU3153 the at least one drill rod also comprises a biased barrier adapted to cover an end of the drill rod when the connection valve is disconnected. In this way contamination control is achieved by preventing large debris ingress by the barrier's close fit around the end of the drill rod. 5 According to a second aspect of the present invention there is provided a drilling apparatus comprising: * at least one drill rod comprising: o a connection valve for connection to a connection valve of a like drill rod or a connection valve of a hydraulically powered hammer; 10 wherein * the at least one drill rod also comprises o a biased barrier adapted to cover an end of the drill rod when the connection valve is disconnected; and o at least one flushing medium line for supply of pressurised flushing 15 medium proximal to the connection valve. In this way fine particle ingress into the drill rod is also prevented by the pressurised flushing medium being fed in a controlled manner from the inside of the drill rod to the outside environment thus carrying harmful dust particles away from the disconnected drill rod connection valve. 20 Preferably, the drilling apparatus also comprises at least one filter drill rod in fluid communication with the pressure oil line. According to a first aspect of the present invention there is provided a method of 3 James & Wells Intellectual Property Ref: 234270AU3153 using a drilling apparatus, the drilling apparatus comprising the steps: a. moving a drive system comprising the barrier along an external length of the drill rod until the barrier it is proximal to an end of the drill rod; b. disconnecting a drill rod at a connection valve from a like drill rod or a 5 connection valve of a hydraulically powered hammer; c. moving a barrier to a position to cover the end of the drill rod; and d. activating the flow of flushing medium from at least one flushing medium line proximal to the connection valve to prevent ingress of dust particles into the drill rod. 10 BRIEF DESCRIPTiON OF DRAWINGS Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which: Figure 1A shows a top view of a preferred embodiment of the drilling apparatus 15 of the present invention in the form of a rod connection; Figure 1 B shows a front view of the preferred embodiment of the present invention shown in Figure 1A; Figure 1C shows a side view of the preferred embodiment of the present invention shown in Figure 1A; 20 Figure 2 shows a sectional view of the preferred embodiment of the present invention shown in Figure 1A with two adjacent drill rods connected; Figure 3 shows a sectional view of the preferred embodiment of the present 4 James & Wells Intellectual Property Ref: 234270AU3/53 invention shown in Figure 1A with two adjacent drill rods disconnected; Figure 4 shows a sectional view of another preferred embodiment of the present invention in the form of a filter rod; and 5 Figure 5 shows a sectional view of the preferred embodiments of the present invention shown in Figures 1A and 4. BEST MODES FOR CARRYING OUT THE INVENTION The invention is now described in relation to one preferred embodiment as shown in Figures 1 to 5. 10 Figures 1A to 3 show a barrier in the form of a sliding door 1 which is open during drilling, but which is closed when the connection valve 2 of a drill rod 3 (best seen on Figures 2 and 3) is separated from an adjacent connection valve 4 of another component of the drilling apparatus 100 such as another drill rod 5. In this way, oil 15 within the drill rod 3 is protected from contamination from large rock debris when the connection valve 2 is exposed. The sliding door 1 is mounted to a pair of actuators in the form of cylinders 6A, 6B. which are adapted to move the barrier 1 in the direction of arrow A (shown on figure 2) in a plane perpendicular to the plane of an end of a drill rod 2 from the 20 open to the closed position. The lateral displacement of the barrier 1 during movement in the direction of arrow A is limited by the barrels (6C, 6D) of the cylinders 6A, 6B and end stops 7A, 7B. The barrier 1 is mounted on a sleeve 8 (best seen in Figure 2) adapted to fit over the external surface of the drill rod 3 and is movable up and down the drill rod 3. 5 James & Wells Intellectual Property Ref: 234270AU3/53 The movement of the sleeve 8 over the drill rod 3 may be achieved via an external drive system, such as a hydraulic cylinder (not shown). A short linkage (not shown) back to the drilling rig mast or breakout table prevents the sleeve 8 from turning with the drill rods 5, 3 during drilling. 5 Figure 2 shows the sleeve 8 in place during drilling with the sliding door 1 open. In use, when a drill rod 3, 5 is to be added or removed from the drilling apparatus 100 the sleeve 8 is moved into position over the joint between the drill rods 3, 5 and then the drill rods 3, 5 are disconnected. Once the drill rods 3, 5 are separated the sliding door 1 is closed and the sleeve 8 is fed pressurised flushing medium, in the 10 form of air, via nozzle 9 and until the connection between adjacent drill rods 3. 5 is ready to be re-established. Figure 3 shows the sleeve 8 positioned over an end of the drill rod 3 with the sliding door I closed thereby covering the connection valve 2 of the drill rod 3. The flushing medium flow from the inside of the drill rod 3 to the outside 15 environment prevents fine particle ingress into the drill rod by carrying harmful dust particles away from the disconnected connection valve 2. The sleeve 8 also comprises a replaceable brush section that cleans the drill rods 3,5 as the sleeve 8 is moved over up and down the drill rod 3,5. Given the harsh underground environment that DTH drilling apparatus 100 has to 20 operate in there will always be a certain amount of contamination introduced by the rod handling process- As a further measure to protect the oil extracted by the hammer 10 (as shown in Figure 5) and other system components at least one pressure oil filter 11 is used. The filter 11 is in the form of a filter drill rod section 12 which is connected in-line to the apparatus 100 (preferably behind at the top of the 25 hammer 10 as shown in figure 5). 6 James & Wells Intellectual Property Ref: 234270AU3/53 Figure 4 shows a sectional view of the filter drill rod section 12. This filter drill rod section 12 has a first 13 and second 14 connection valve at either end compatible with those of the drill rod 3, 5 and the hammer 10. These connection valves 13, 14 screw into either end of a reversible wear sleeve 15 thereby 'sandwiching' the filter 5 elements 11 and their housings between opposing threads 16 of the sleeve 15. In use, the filter drill rod section 12 would be unscrewed and replaced with another one periodically during drilling. This change of filter drill rod section 12 could be implemented within a few minutes and would create minimal disruption to production. The filter drill rod section 12 would then be disassembled in a clean 10 environment away from the work area and the standard (off the shelf) hydraulic filter elements 11 replaced with clean ones. The filter drill rod section 12 would then be delivered back to the drilling rig for reuse as required. Thus, preferred embodiments of the present invention may have a number of advantages over the prior art which can include: 15 e failsafe contamination protection of oil from drilling debris (cuttings); * failsafe contamination protection of cuttings from oil (important in mineral sampling applications); and * improved reliability through prolonged service life and consequent reduced maintenance costs 20 * Low production cost as a result of modular design and reversible drill casing. Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims. 7