The Auster was likely to have been AOP9 XN435 which, according to my records, was with the Hestton CCF at the Speedbird Club from around August 1973. It was allocated the registration G-BGBU 8.11.78. The registration was cancelled in 1990 by the CAA but the frame may still be stored somewhere.
Thanks edskarf - you have no idea how many people I have asked over the years. Yours is the first confirmation of its identity.