Potterton Prima F80 cold start problems ?

My boiler fails to start from cold but I can get it working by turning down the boiler stat to an almost off level and powering back up. At this point the boiler enters it's start up sequence and starts heating and will correctly stop when it reaches the low tempreature.

Once warm it continues to operate when I turn up the boiler stat to normal but this means that it wont start the next day.

It is obvoiously a boiler stat or control board problem but which?