I’ve got a Halstead boiler as well and I’ve been having problems with it for over a year – ignition and overheating cut out. Got a plumber in and all he wanted to do was fit a new boiler so I went on to:
http://www.ukplumbersforums.co.uk
One of the suggestions was to replace the thermistor. While comparing the picture of a thermistor on ebay to mine I discovered it was loose (as well as the overheating thermistor) – tightened it up and problem is now sorted. OK it’s unlikely your thermistors will be loose as well but it’s worth a check.