my pilot light wont stay on baxi solo pfl 30/80 boiler?

i have recently drained down my system to replace a radiator. after refilling the system 1.5 bar pressure. I have made sure that the sytem controls are on and calling for heat themostat etc. The overheat light is displayed on the control panel, Ihave turned the control knob anti clockwise as it indicates on the boiler panel, waited 10 seconds and turned it back on.all lights come on in the panel, the pilot light stays on for a few seconds then goes out. I can also hear what sounds like the tank refilling althogh all taps are running and pressure is at 1.5 bar. please help