I have a Yamaha Virago XV125 on a Y plate, this gives the year 2001, but the V5C says the date of first reg is 2000 and DVSA say that it's 2001, who do I trust for this??
(I'm sure Yamaha stopped making them in 2000...)
'Y' registrations in the UK ran from 1st March to 31st August 2001. It's quite likely that a vehicle registered in that period would have been built in 2000, as vehicles often sit for a while in storage or in the dealership before being sold - especially ones like that at the end of manufacture - they don't sell well as most people would want the replacement model.
As DucatiKiller says, the VIN should tell you the actual year of manufacture, and that's the most trustworthy of the figures. The V5C usually gives the full date of first registration IIRC - what is the rest of it?
If you are purchasing components for the vehicle you will want to base it upon the VIN Number. Here is a UK VIN number decoder.
Best of luck