Pete, I had the same sort of problem with an old motor bike that I bought and the Seller had lost the documents.
You need to write a letter to the DVLA. Give them the registration (I presume you know this?)
and inform them you have lost the Registration document and need a new one.
You did say 'Get it Back on the road) so they should have all the related info (Engine Number, make/model etc) on their Database.
If the car was registered correctly and all the other info is genuine, you should get a new V5 (I'm not sure if this will incur any cost?)
However... If the Car has never been registered or on the road since it was built, then you may need to go down the SVA route

And this may lead to a 'Q' Plate if you dont have the original Donor documents.
Hope this helps.
Col (Taunton)