potterton 100e profile boiler?

Hi I have a potterton profile 100e fanned balanced flue gas fired boiler.
my boiler works fine 75% of the time but when it is extremely cold it cuts out and I have to keep pressing pilot button. it comes on for 15mins or so and then cuts out. Sometimes it works in the morning other times we wake up to a freezing house. On mild days we have no problems. It always happens in late December and early January. Help is it the circuit or thermo value? It has been working fien this winter until today again!