Worcester 240 Tripping Gas Switch?

Guys,

Got a Worcester 240, the pilot lights and remains lit UNTIL the radiators kick into action, at which stage every time within 3/4 minutes the grey gas valve button clicks out, I then have to try to relight the pilot, takes time but eventually it relights...radiator heating kicks in and once again it trips!!

Anyone any ideas?

All radiators are air free (bleaded), and all fully open on the thermostat, the pressure is at around 1.8bar on the meter.

Cheers, Ash