Vaillant ecotec plus 831, F75, No heating, Diverter stuck?

[Technoprobe]

Hi,
I had this 831 plus installed 3 years ago by a good plumber who thoroughly flushed and added inhibitor to the system.
I have had the notorious F.75 codes since year 1 and had to trip the sensor via the filler loop on numerous occasions to get the heating going. Eventually it gave up completely and I had the orange pressure sensor swapped for the improved black one.
It worked for about a year (still throwing F.75 codes) and the heating has packed up again.
I replaced the sensor this morning and the heating fired up successfully all the rads got hot and I went to work happy :teeth_smile:
Got home and its broke again 🙁

The heating fires up, quickly reaches 80 degrees and goes in to anti cycling followed by constant overun.
The flow and return are not heating up outside the boiler but the pump sounds like it's running.

If I look with a torch into the slot in the diverter I can't see the pin moving when switching the hot kitchen tap on and off. Should it visibly move?

Here's a vid of the part in question (I can see some brown deposit through the slot too)

youtube.com/watch?v=E5DMjr5cgeQ
Sorry, no URL's in first post 🙁

Thanks for any help.

 

[Dotty]

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