Alpha 240E - pilot light goes out?

When the heating is switched on at the boiler the pilot light ignites and keeps going for about 2 hours or so, then goes out and wont re-ignite, but the boiler continues to pump the (now unheated) water around.
Once this has happened it takes several hours (around 8) before the system will come back alive.
Any body got any ideas,

Thanks in advance