Trouble with DHW - Heating OK. Potterton 30HE

I have a Potterton Performa 30 HE Boiler. Heating works fine but DHW is lukewarm. If I turn the selector switch to DHW only and then turn on a hot tap, the DHW outlet pipe from the boiler is warm but the Heating Flow pipe gets very hot.
When the DHW is running both of the diaphragms in the Diverter Valve operate, I can see the push rods switching on the Micro-Switches.
I would very much appreciate advice on the possible cause.

The rear part of the diverter valve isn't moving properly. They get "crudded up". With a van full of parts one can refurb them, but it's easier to replace them as several pieces age. Part is 248061 iirc.

ChrisR said:

The rear part of the diverter valve isn't moving properly. They get "crudded up". With a van full of parts one can refurb them, but it's easier to replace them as several pieces age. Part is 248061 iirc.

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Thanks very much Chris, now that I know it's the Diverter I can sort it out.