Gen,
you still don't mention which Potterton boiler you have.
From what I have read on here B Gas have some pretty shabby "upselling" tricks - they may want the business of puting in a new boiler at top-dollar prices. Seek another opinion before agreeing to any proposal they make.
And when he says, "they don't do the parts any more" does he mean just BGas or the hundreds of UK merchants who sell boiler parts I wonder?
Peccavi, February 2009