Can't help directly, Simon, but I have experienced this sort of problem before regarding serial numbers related to lost or active aircraft. One suggestion is to first double-check the, presumably handwritten, entry in the log book, just in case the serial number might appear to read as you have quoted; it may be, for example, that the figure 5 could actually be a figure 8, and so on, if you see what I mean. Given that the log book entry is correct, then the only thing I can recommend is to try the Public Records Office at Kew, where you will be able to access the relevant Record Card for the aircraft, as well as the Squadron diaries and Operations Log. I'm afraid I can't help you much on the procedure for this, as I have used a friend who lived in London to do the work for me, as I am in the north! However, I know that it is much easier and quicker nowadays, due to the computerisation of indexes (indices?) and the Freedom of Information Act etc.
Hope you find what you are looking for, good luck!