If you have got a red cylinder near your boiler it could be a sealed system, is there a pressure guage near it and if so the pointer should be at 1 - 1.5 bar
If the pilot is staying on then the fan must also continue to run which meens it is not the stat or aps, the most likely cause is the solenoid on the gas valve dropping out.
The vokera option has a water pressure gauge behind the outer case.
If the water pressure in the system drops below 1 bar the amber light comes on to warn the the pressure has dropped, once you top up the system above 1 bar the light goes out. If the pressure drops below 0.5 bar the red light...