This boiler is about 13 years old and has been very reliable until now. It ignites first time, stays on for about 5 minutes then switches off even if it has not reached the correct temperature then tries to relight. Striker has numerous attempts to relight but either fails to light or lights after about six attempts. It then goes off again. Had new gas valve fitted a couple of months ago, cleaned out and tried new heating system programmer but hasn't worked. Any ideas? Could it be a faulty boiler thermostat or perhaps the ignition control box? Help!!