hi,i had the same problem with my potterton boiler it was the printed circuit board (pcb) i got a rebfurbished unit from cet ltd their site is-http://www.cetltd.com/terms.asp the unit only cost me 30 pounds as i picked item up in person but you can have the unit sent to you at a cost of 8 pounds but you do need to send your old unit back to them this is part of the terms and conditions , hope this helps rob.
rob
January 2006