Hi! Not sure this is relevant anymore, but for anyone else looking for a solution to a similar problem here is what worked for me on my Potterton Powermax HE 85.
I had no display while the mains light was on (green)
I called Potterton and they said that it must mean a fault on the circuit board.
I decided to get my plumber to have a look at it before splashing £200-300 on a new board. He identified the circuit board fuses to be the problem. They had actually burned out. He replaced them and the boiler came back from the dead (at a fraction of the cost)!
FYI, there are two fuses on the circuit board to protect it from power surges. The fuses are encased in two black screw-like sockets at the lower right corner of the back of the board (when facing upwards).
Having no display does not mean that the board needs replacing necessarily as it turned out in my case, so check it out and possibly save loads!!
I would recommend getting a CORGI certified plumber to replace them if you ever need to. It is quite a delicate job and you need to connect all the sockets back properly.
If despite the above it is the board that needs replacing the part number is 5107535.
Hope this helps
Chris
February 2009