You may get one from Potterton but if it is not working most parts if avalable will be very very costly even at trade
price and you still need a corgi registered person to fit them.Sadly I think it's time to change it.When new it was a class unit 25 years on it is past it's sell bye date.
retired fitter over 30 years in the trade.
January 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.