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 myself…and only then with the help of this board.
See link
http://www.diynot.com/forums/plumbing/vaillant-issues.303035/#2214404
In terms of cost you need to be thinking about:
• Powerflush
• Boiler
• Temp valves for radiators if you don’t have already
• New cylinder
You’ll be lucky to get that done for less than £4k.