Cornet - I'd work on an AVERAGE of £3500 per month. It all depends how hard you want to work.... In my first 6 months on the line I flew 100 hrs more than some of my colleagues (and therefore earned more money!).
If you have lots of leave, or are on reserve and not used much you may take home only £3000, this month I've done A LOT of flying, and an overtime trip, and will take home after tax £4700. I, however, can't remember where my home is this month!
A normal month of full flying, no leave should see you with £3700-800.