F13 can be attributed to the wiring harness or either of the pcb's. It is generally a connection issue between them.
I would recommend that you contact Group Service, the service division of Glow-worm as listed in your copy of the instructions.
If does sound like you state that the product is out of warranty, this would make the visit chargeable. However you can be sure that the correct part is replaced and the work is guaranteed.
All the work listed above does require that it is undertaken only by a Gas Safe registered engineer in Great Britain.
David Parker (Product Manager for Glow-worm)
David Parker
August 2009