Ian,
Have a read of CAP371 'Avoidance of Fatigue in Aircrew'. You'll find it at
http://www.caa.co.uk/docs/33/CAP371.PDF
CAP371 is the FTL document which defines maximum duty hours, flying hours and commanders discretion. Although each company might have a slightly different interpretation, the limits are printed here in black and white, and any UK scheduled operation will have to adhere to them very strictly. We all face CAA audits on a regular basis, and CAP371 compliance is something that gets looked at very closely.
This is the gospel according to Gatwick. That is of course assuming that you are operating a UK registered aircraft with a UK license and a UK AOC (Air Operators Certificate).
I think one other area to be considered when looking at fatigue is roster turbulence/instability.
The LoCo I work for flies me for approx 870 hours every year. Last year, crewing and the rostering system were in turmoil, and my 870 hours almost finished me off. This year, however, crewing and rostering have been sorted out, and I'm flying exactly what I was rostered for. I'm actually on track to do slightly more hours this year, but feel a lot better for it ...
Good luck ...