Thing is HD, we don't actually get overtime per se. All that happens is they buy a day's leave off you. For me (after Gordon Brown's had his slice) it comes in at around £63; hardly worth making a mess of your leave for - so I too generally refuse to do any "overtime". An easy way to make cash it is not; generally only a couple of days a year will come your way (if you're interested).
I think the figure mentioned for AVAs at Swanwick was £500 per day. We have been told categorically that this will not apply to us (nor I suspect at any airports).