You should be entitled to a refund in the event of cancellation in line with EU Regulation 261/2004:
http://www.caa.co.uk/default.aspx?ca...90&pageid=4409
I had this recently on a bmi flight (last of the day) cancelled when the aircraft went tech and had to fly the following morning instead. Got a refund of 250 Euros (about £160) when I wrote to them quoting the regulation. Not bad for a £25 flight. Made me wonder though, how many passengers in the same boat did not bother to seek the refund to which they are entitled.