As someone who also had a Potterton boiler last year...and decided to changeout I wholeheartedly agree with some posters to keep the boiler if at all possible.
I thought I was doing the right thing going for Vaillant but ended up with a nupty installer and spent weeks sorting out the issues...
Struth! As a new person on the board about to order a new boiler I'm amazed to see extent of the problems with this system.
Looks as though I won't be buying a Vaillant!