Just a tip for those who have these units. Mine packed in during the very cold Christmas night. I had noticed that the fan had been sounding a 'bit' noisy and when outside I sometimes heard a faint squealing noise from the exhaust. When I got the service engineer in (David Leslie of Berwick upon Tweed) I admitted to hearing that noise which most people may not have admitted to. He checked the unit over and concluded that it was either the fan or the 'board' and promised a fix next AM when he could get the parts. Of course I spent the night worrying about all the other horrible stories I have read here. I haven't got the bill yet but I am sure it will be reasonable and most of it could have been avoided had I not ignored what the machine tried to tell me. I could have stuck the fan in myself without a Bank Holiday service call.
I have posted this for 2 reasons. (1) Know and use your local engineers. They work for you--not some big empire in the sky.
(2) Tell the man EVERYTHING you know about the problem. Don't give him a puzzle to work out.
And if you have a Potterton--the troubleshooting 'Logic Tree' is laminated inside the panel you remove to see the guts of the unit.
Happy New Year.