Gatwick - Summer 80-100, winter 30-50 - Annual 800-850
Luton - Summer 70-100, winter 30-50 - Annual 750-800
Stansted - Summer 50-80, winter 20-50 - Annual 650-700
Smaller bases tend to be much quieter for obvious reasons.
That's based on the guys I trained with at these bases. Gatwick guys tend to get hammered. Doesn't seem to be any hard and fast rules though. I'm on a perm contract and averaged 45/month during winter at Luton, some guys on flexi had 80-90. But also vice versa, some flexi guys complaining about not having many hours and perm guys getting hammered. Swings and roundabouts is the name of the game.