Here's a link to a guy repairing a 407677...
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
You can buy one from heating engineers' merchants and expect to pay £130 or so... Some places offer re-con PCBs but on an exchange basis - you send yours in first.
Here's an internet supplier selling for £113.49 plus carriage...
http://www.uk-plumbing.com/potterton-8407677-pcb-p-24989.html
Good luck...
Peccavi, August 2009