Ok here is a lesson for anybody assuming the pcb has died. This is just what I did and nearly spent £125 on a new one. In my case the boiler suddenly came on to central heating after a power cut. There was no way to turn it off other than to switch it off at the plug. It was assumed the power cut spiked the pcb as the control panel had no effect whatsoever on the boiler.
The culprid was the micro switch located within the box attached to the end of the diverted valve. this had a black and white wire leading to the pcb.
Remove the switch by pulling the metal retaining clip out then remove the entire switch and the cable from the pcb box.
Gently prise open the case there are three little lugs on each side. Inside you will find a small micro switch and a rocker make sure you dont loose the rocker and take a note of how it sits.
If you have a multi meter connect up the ends of the black and white wire for continuation. If the meter gives a reading its the contacts of the switch that have stuck together. A gentle tap with a hard object may be enough to dislodge the contacts if this works a spray of wd40 or contact cleaner will sort it out.
Micro switches are two a penny so if its shot you can buy one and replace it . Put it all back together and you should be cooking on gas. I nearly signed a contract for a replacement boiler costing £2k so this fix may save you megga bucks
Hope this helped
in this case i would be looking for something similar starting at the diverter valve and see if it has a switch box attached or a micro switch which could be the culprid.
Hope this helps as it sound very similar to my problem with a Jaguar/glow worm /protherm make boiler
tom
July 2009