We used Dermarta http://www.dermarta.co.uk/
for our loft extension (part of big overall building project finished just before Xmas) which turned out brilliantly. They're based in Tooting. Only thing is I think Dermot, the head honcho is away at the moment for a couple of weeks, but he might be picking up emails and/or his foreman, Les, would be on to it.
Dermot introduced us to an architect called Mike Robinson who did a great job and also sorted out all the planning regulations and whatnot.