All towbars now have to be type approved, and car manufacturers by law have to supply the dimensions of where the tow kits have to fit so you shouldn't (bar shoddy workmanship on the tow kits manufacturers side) have a problem fitting the parts.
Redhand is correct in saying that all towbars fitted to vehicles after 1st Aug 98 have to be type approved (mine is a 99).
The rest of my statement still stands!