Get yourself the installation instructions from the potterton website, that has decent exploded diagrams. The DHW pressure switch is on the front of the diverter valve, you will need to remove the front panel to get to it (you can see it from underneath). Remove the microswitch, sensing pipes and grub screws holding it onto the diverter and it will be free to unscrew the two halves to reveal the diaphragm assembly. Make sure you close the boiler taps, drain down and isolate leccy before doing this.
Be interested to know if you fixed it and where you got the part from as mine did the same thing on xmas day.
Big D
December 2008
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