As Dave states, without pictures of the car, it's difficult to make a decent assessment of it's restorability. Whether to keep the car also depends on your long term/short term plans for the car. If you just want to get the car back on the road as quickly as you can. This is how I would attack the problem and just my opinion.
The first item to check is the chassis. If it's rotted, plan on buying a new one. Being in the UK, you have the advantage of buying a new one from a dealer, Banks or Spyder. Chassis replacement is not that difficult, it's just time consuming and a pain in the ass and probably expensive.
The second item is the engine. If it's seized up, time for a tear down and rebuild. The Renault crossflows as a replacement may be more plentiful on your side of the ocean and cheaper than a rebuild of the original.
The third items are just checking/replacing/rebuilding all electrical brake, suspension to make sure everything works well enough to pass your safety inspections.
Bodywork, you can work on it after you get the car on the road (don't know how the UK motor vehicle regulations are). The last unknown is the missing V5. If it's the same as a legal title of ownership here in the US, this car may just end up being a parts car.
In ending, without seeing any pictures, you probably have a time consuming and expensive task ahead of you. How committed are you? I would talk to Dave first.
Joji Tokumoto
Fallbrook, Ca