How can I fix my Potterton 80e bolier that keeps tripping the reset button?

In this cold weather the boiler operates fine up to a point and the reset button is triggered. Meaning when it gets cold in the house I check and have to press the resst button to fire up again.
Example - this morning timer was set, thermostat set to 4 on boiler, 20 on house, so at 7.30 heating and water comes on fine but after 40 minutes (approx) and before temperature in house is reached - boiler cuts out and reset button is needed to fire up again.
Got my 89 year old mum coming for Christmas and must have heating working!
Boiler was in the house from new (18 years old now), had issues years ago (all fixed at time) and serviced intermittently.