Thank you both very much for your advice. And thank you, Stuart, for your kind offer. I do already have authorisation to acquire a long barreled pistol, so it seems that now I have to play a waiting game. I believe I saw one (make unknown) come up on Guntrader in the last few months - although I didn't have the opportunity to buy at the time. Sod's law, eh?
If anyone else has a tip as to where I might find one, or perhaps even have one of their own that's gathering dust - I'd love to hear about it.