Thanks for all the advice so far. I think this is a combination of presenting an overwhelming amount of evidence, letters from Ducati, et al, keep the original numberplate and of course I have the plaque which is a "Certifcato di Autenticita", so that'll add a bit of weight. Fingers crossed. Will keep you updated.
Update. Well, I appear to be solving this puzzle. Customer Service Ducati ITALY rang me up after I had connected with a grown-up in Ducati Motor Holding Spa on LinkedIn (a useful tool that tends to generate a bit of traction when solving problems). They were exceptionally helpful and have said the Homologation Office are working on my request and will send me a Technical Sheet attesting that the bike is original. The agent in Cyprus now said that Cypriot customs will also register the bike if accompanied by the original DVLA V5 (which I have). So it looks like I have closed the loop at both ends - wish me luck and thanks for all the advice.