RHD one as well. How much rot was there?
There was very little rot, the car had stood twelve years in a small garage before I bought it. I decided to clean the underside, and in places there was some surface rust just starting so I think I caught it just right, I can't find any previous welding and I believe it is all original metal under there, There was some minor rust around the flanks in places and we did put a couple of lower panels on but that's all, the sills and doors are all good.
I purchased a blasting pot and cleaned the complete bottom of the car, then treated it with Bilt Hamber rust converter just to make sure, then three coats of two pack epoxy , then an under seal.