How much does a Diverter Valve and Cartridge cost for a Potterton Powermax HE 85 CP?

I posted this question in September: https://www.diynot.com/diy/threads/diverter-valve.509393/

The answerer talked about diverter valves and cartridges. I have a few questions:

1) What the difference is between a diverter valve and a cartridge?
2) Why could the cartridge fail independently of the diverter valve?
3) Where can I buy a diverter Valve for a Potterton Powermax HE 85 CP (a hyperlink to an advertisement would really help)?
4) Where can I buy a cartridge for a Potterton Powermax HE 85 CP (a hyperlink to an advertisement would really help)?

The reason I ask is because I recently requested three quotes from three local plumbers, which ranged from £150-£400. Therefore I am trying to understand how much the parts cost first.

 

Multiple threads don't help.

 

Three plumbers time you wasted already then.Picked their brains to diagnose the fault,then buying the parts yourself,just love sneeky customers.

 

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