You can get limited access by opening up the wheel arch liners at the edge by the sill. I put the pressure hose in there and blasted all the crud until it ran clear from the drain holes. I left it a couple of weeks (in the summer) to dry out, used a heat gun along the sill to make sure that there was no moisture left, then sprayed internally via the 360 degree lance supplied and through the drain holes / jack pad apertures / end access holes mentioned previously with this stuff:-
Mike