From description of the leak its the joint between the pan and cistern.
The holes I've no doubt are screw holes to fix pan to floor. Those holes [usually two each side] should have chrome or brass screws in them if not job not complete.
Get back those that did it. B&Q if they supplied the fitter as well. Don't try repairs yourself. And insist on the pan being screwed to the floor with rust proof screws.Post how you got on.
Nobby
November 2008