How to stop Potterton Back Boiler going to Lock Out?

Hi

I moved into my present house several years ago and found that the central
heating system is provided by a Potterton Housewarmer 55 Back Boiler, with
a Potterton Stratton Inset Gas Fire at the front.

Ever since I have lived here I have had a constant problem with the
backboiler. Once a day (almost every day) at some stage the boiler goes to
lockout and the red indicator is illuminated on the side of the gas fire. I
press the reset button and the fire comes on, and usually continues okay
for the rest of the day. Sometimes it goes off more than once a day.

The problem is, if we are away from home, the heating will remain off until
we return, and this can be a danger in very cold conditions.

I have had British Gas engineers in to look at the boiler on numerous
occassions and they fiddle about with it but the problem always returns.
They really seem to know what is causing the problem.