ch overheating, why?

The CH on an apollo is overheating. Initially there was a grumbling that started in the airing cupboard and then it would progress through the house. Since the plumbers been the rumbling has stopped but the boiler gets to temperature and completely shuts down, but only sometimes. Sometimes it works alright. Is this to do with the overheat stat or is it a gas valve problem?

andy
March 2007