Well - did lots of trawling of the CAA and Eurocontrol's websites and although mention is made of Mode S designated airspace, I'm damned if I can find any directory or map where it is defined. There is a vague inference that it is presently only required in busy terminal areas.
So I'm of he opinion that if my ATC flight plan says mode C, then the ball is in their court; I've done all I can do to make sure I'm legal.