I was in the same predicament here, chassis, but no number, So I contacted my local DVLA centre, told them that i had bought a part built kit off a friend who has since died (sob) and that I couldn't find the chassis number anywhere. Within 2 days I had a letter with a new VIN number on that just needs verifying by a garage with an authentic stamp, (my Mate has his own garage so not problem there)
Put you request in writing ans the DVLA call centre was totallty useless, but the people in the local office seem to be more clued up.
I hope this helps.
by the way approximately 3 weeks away from being SVA ready