Luke, cheers for the tip, that looks about the right price for me! I might give it a go.
I've finished the bracket for my camcorder now, total price was about £15 including a microphone. I got a U bolt from B&Q (£1.50) that was a bit bigger than the rollbar, and drilled a couple of holes into a large right angle bracket (from the decking section in B&Q), cost about £2. I bought a metal universal tripod adapter from Jessops (£7) and rivetted this to the top of the bracket. I wrapped a lump of rubber (part of an Escort cam belt, I think) around the rollbar and mounted it on one of the rear bars on the FIA rollbar. Very secure, held firm by a bit of insulation tape and long cable ties. The microphone (Maplins, £4) is taped to the inside of the car, everywhere I put it seems to give loads of transmission noise though.
Very easy to do, about 30 minutes work and £15 to get it right, and it only take 5 minutes to fit.