Finally sent my V5 off to udpate after my 200tdi conversion... after a month and a phone call I get a letter requesting loads more info, proof, receipts, etc. I didn't think a month was a very good response time...
So i emailed their complaints section. 24 hours later i've got an apology and my new V5 is in the post to me with no further action required.

Apparently the person looking into my complaint realised it was a historical vehicle so it makes no difference what engine it has in

Just thought i'd share that really in case anyone else has a similar problem in the future

Yep that's right. They decided that pre 73 and you DON'T need garage letter headed paper etc. Mine is still in the works, must chase them...!