I have no link I'm afraid, it was on the television news, not sure which channel, heck I can't even remember what day it was; it may have been after the debate. But, as I understand it, there is no confirmation that Cameron has actually agreed.
---------- Post added at 21:14 ---------- Previous post was at 21:06 ----------
Just found some stuff about Brown agreeing on Friday to Paxman interview.
http://www.politicshome.com/uk/story/4083/
P.S. I have not had any to time to vet the source, I have no idea who they are (I am sure someone will find a bias somewhere), but it does seem to indicate that Cameron may be just jumping into it, even though many a Tory wishes he wouldn't.