As another project, I'm considering scratch building a chopper. Do bikes have to under go the same sort of thing as a car? Is there a separate SVA manual for bikes?

well there is now the msva but if you are building a bike from scratch.....get an old steel frame with a log book from a breaker....use the headstock in your new frame and bingo no need for any silly tests other than MOT.....get an old log book and you could end up tax exempt....

Sounds good :] But won't the V5 state the model and dtails of the bike? Wouldn't I need one that stated it as being "custom" or "spare parts" ?

No as am sure bike v5's don't really give much info other than make,colour and engine size.
I built a streetfighter using a 1981 DT125 headstock but the engine was 1989 FZR600.......no problem with MOT/insurance.

MSVA for bikes must be a complete nightmare.........just think, it's got a chainsaw at the back!!!

Might it be that older (1980s) documents have less details? As I'm pretty sure more recent ones are quite specific.