Thanks Karl, the pictures are a real help !
More on the floor .....
We have cut off the cill on one side and cut an inspection hole on the other with a view to understanding exactly how the floor was made so that it can be remade accurately.
As previously thought, the floor sandwich comes as far as the S shaped chassis rail and then the outer section is welded on afterwards along with the outriggers. There are box sections at the front of the chassis that incorporate the bulkhead support, but the rear box section that supports the door pillar, appears to have been added with the bodywork ie by the coachbuilder. Once I have clearer photos I will post them
The photo shows the position of the internal cross members and the outriggers supporting the bodywork. The chassis rails are remarkably good, only one small section to replace. Then a single,long repair piece to replace the lower floor where it is welded to the chassis rail. Once the front and rear box sections are tidied up, then the new outer cill can be added
Looking at the construction, the cause of "problems" originally was that water is allowed into the front of the cill from a gap inside the engine bay - so we will make a small mod to prevent this happening in the future !