http://www.bikecyclingreviews.com/708replacing_a_cassette.html
That might help
Says you need a 16 mm spanner..
I found the simplest way was to take my wheel to the local bike shop - he released in a few seconds and I gave him £2 for his coffee tin
is that small bike shop in Baker Street (near the level) still going?
maybe cheaper than most palces I can think of, right now..
otherwise, have you tried popping along to the recycling(!) place, in Lewes Rd- magpie?