Worcester bosch 240 Combi BF & OH pilot light extinguishes after an hour?

The pilot light extinguishes after about an hour on heating, shorter if ignited soon after it's gone out. The pump continues to run if heating left on or hot water demanded after extinguishing. Pump stops after about 20 mins when heating turned off normally. Works fine for hot water if heating is left off. Changed thermocouple and thermostat, fault remains. Any help would be appreciated