Keston C36 pump overrun problem



I have a Keston C36 running on Summer Time setting.

If DHW is called for (i.e if I have a shower), the boiler runs perfectly.
AFTER the DHW demand, however, the boiler stops firing and goes into what I assume is the pump overrun state, but... the water temperature reading increases to around 83 degrees and the boiler does not shut down (I would expect overrun to last about 5-10 minutes?? - this lasts much longer than that) . I have to manually switch the boiler off and back on at the external isolation switch.
When I switch it back on at the isolation switch, you can hear the hot water being diverted and the water temperature rapidly falls to around 30 degrees and the boiler shuts down within about 15-20 seconds.

From reading other posts with similar symptoms, I'm thinking it could it be a faulty diverter valve or (hopefully) a simple replacement capacitor for the diverter valve?

I obviously need to get someone in to sort it out but I am hoping someone can:
a) Tell me where the hot water should be getting diverted to after DHW demand.
b) Diagnose the problematic part so that I can hopefully cut my call-out charge in half.

Installer settings (in standby):
1. 000
2. 045
3. 034
4. 035
5. 1.2
6. 007
7. 035
8. 000
9. 082

DHW set at 65, CH at 80

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