A big dilemma CP!! You don't want to price yourself out of the market by asking far too much but you also don't want to appear cheap and annoy fellow musician in the area by appearing to undercut them....If I remember right your in the London area so you'll be charging more than most places in the country anyway....You'll have to take into account the following...the size of the venue, without being cheeky....are you any good? (if it's a first gig explain and give a reduced rate perhaps?), is there the possiblity of more work in the same place? (again a reduced rate for follow-on gigs could be arranged)....A better bet might be to ask in your local music shop as there's always at least one gigging muso on the staff there and ask them what the going rates are......If all else fails what the heck ask for �200 and you've got a ton in your pocket......and it give some room to haggle.