Just send a small letter with the V5 explaining who you are and your address blah blah and the situation. You'll be fine mate, one thing though the sooner you do it the better it'll probably look.
All thats happened is the chap you brought it off has never been the 'registered keeper' for whatever reason. I wouldnt worry about it. If you are in possesion of the whole V5 just fill out all the new keeper details (obviously now you) and just forge the signature of the seller and send it off.You'll be fine mate. ;D
Ohhhh, didnt I see this car in Practical Classics a couple of months back? Also wasnt it at Billing 2007 this year, I remember seeing it in the Convertible Car Club show stand? Very smart ;D