Sharon,
The folowing site www.partsarena.com will give you detailed exploded views of many boilers including Potterton ; manuals are also included. This is the Baxi site and they own Potterton amongst other boilers.
Your problem however appears to be related to the external controls, the priciples of which are covered in the manual. The best thing is to get the manual on the programmer and boiler. Then find out out what the HW/CH arrangement is on your system. Common one is Honywell "Y" plan as they do many controls ( a bit like all vacumns are called Hoovers). From the wiring diagrams one can usually work out what is happening.
N.B. You will be asked to down load software to enable flash to work.
Perminder
November 2005