Between you and me, Sporty, BG doesn't have a great reputation with boiler servicing.
The roomstat could be faulty. Perhaps it doesn't switch off at the set temp. That would mean the pump is running continuously, trying to get the system to reach an unattainable setting.
Or, quite possibly a simple control problem. A control board fault. (Printed circuit board)
There are various heat sensors that could cause this.
I would suggest you contact Potterton's helpline. What you need from them is a few names of likely installers who know this product. BG are just far too general.
http://www.potterton.co.uk/28/