circuit boards are pretty expensive and an average price for a replacement one and fitting is around 200.
which model of potterton boiler do you have? there is one particular model that you cannot just buy the circuit board, you have to buy the complete control box and then you are looking at around 300.
your best bet is to ring potterton themselves. they offer a service where for 225 they guarantee to repair the boiler and it includes all parts and all labour. in your case this really would be a good deal. when you ring them you have to pay the 225 in advance on your credit or debit card and they then arrange a time to call, the only down side is they only guarantee to call sometime between 8 and 6 i think it is so you have to wait in all day.
please let me know how you get on and what the problem was as i'm a recently qualified heating engineer and its all good experience for me.
p.s. if your in the birmingham area my old man whos also my boss can fix it but potterton are still your best bet.