Think im managing to figure out the problem a little better.
The radiators are heating up, but they then seem to be overheating and im sure this is why the boiler is doing a safety shut off, as it will switch back on but the time it spends working is less due to the fact that the radiators are at a hot temp.
I also beleive that the heating continues to work when the hot water is running because the excess heat is being passed through the running water allowing the central heating to stay on.
Hope this helps you heating gurus to help me in diagnosing the fault accurately.
Cheers!
Neil.
January 2012