Building a hybrid soon.Defender body on disco 1 chassis.How does the law and insurance stand on this..?

chassis is the vehicle designation,and as a hybrid some insurance wont worry others might

Talk to your insurers before you do anything. If necessary change insurers.

As long as you don't alter the chassis (e.g shortening the wheelbase to fit a 90 body) and keep the majority of the rest of the car (I think any two out of the engine, transmission and axles) it can take the registration of the disco. Insurance is then technically a modification to the discovery- albeit a fairly major one. Best to go through a specialist who understands this sort of thing. Once you start hacking the chassis about or using too many non-original components it will go on a Q registration. Theoretically, in a borderline case I think you could do an engine swap into the otherwise unmodified discovery first and get the V5 changed to suit, then do the hybrid conversion.

Personally, I quite fancy doing the opposite- a disco or rangie body on a 130 chassis would make a great double-cab pickup