@Desmophil , @The Royal Maharaja Thanks for your comments. I'm sure the right person will come along eventually. I've put an ad (not an auction) on Car & Classic, which I think goes to Europe as well as the UK. The are several SP3s on there as well as mine, with much higher asking prices. As you say, I'll just have to be patient. I just wish I could ride it, but I really think I'd probably drop it or worse, which would be a pity for all sorts of reasons!
Praise where praise is due. Nice to see an un-molested bike after so many years, notably the OEM silencers, which so many owners removed at the first opportunity and that now ironically fetch a princely sum. ProTwins in Surrey had a still crated SP5 in their showroom for a while, some years back. Lord only knows what that is now worth. Stand your ground and be patient, as many have said. You will eventually marry the bike to the right owner.