Hi chaps
here's some further diagnostic info,

the boiler works fine all day but does not start in the early morning
when all is cold.

however, turning the power supply to it off then on again at the wall
switch, and then using the rotary switch on the front to reset allows
the boiler to start again.

just reset on it's own isn't sufficient.

Any further ideas, I'm thinking its something that's too far apart when
cold but expands to close the gap when warm.

all responses gratefully received.

and thanks to all who've responded so far.

dedics