I have just sent my registration docs away to the Dvla going for a new plate. However I have received an email asking that I fill in a v627/1 build up report... this document asks for a current registration number and info on the donor car, however I have no donor car as all my parts are new, do I just leave these fields blank?

I just filled in what components I used which happened to be a combo of new and used eg Sierra, wilwood, Kawasaki etc.
Your reciepts should clarify what is new.

Yes, just leave that blank