watch out with O2, they have doen it to 3 people i know including myself. They refused to give payment back but credited the statement the following month. Because i had cancelled the DD and sent them the correct figure (taking over payment out) i was then deemed as being in arrears every month and every month when they sent the statement i was cut off the very same day. Each month taking 3 days to reconnect. I was pregnant at the time and need my phone when travelling so i was not best pleased. it took 6 months to sort out the problem and they even tried to take it through the courts.
I got out of my contract in the end, luckily.