Weight isn’t the issue per’se however p/sqI and track pitch are. The track is the same as universal, so pitch isn’t a problem. Paul remade that machine to high standards likely with lighter steel
. , also the surface area regarding track against the road is greater than a universal....ergo it will be well under the legal limit.
What you have is some tool at DVLA who doesn’t know what he is talking about.
Tax / mot exempt, a year and weight (call it 3.5 tonnes)
Go to another office and try again mate.