Basically the important point is this:
"Due to bands cancelling at very short notice and bands promising a big crowd and then bringing basically nobody, a £50 deposit cheque would be required. This will be uncashed and returned on the night to you. The only time I would cash it, is if you cancel within 6 weeks of the gig date or you bring less than 20 on a Friday / saturday night! If you're a genuine, hard working band, then you should have no problem with this."