Replacing a pressure relief valve

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This is my hot water tank. The cold water supply pipe comes in behind the tanks and loops up and over to come in at the top of this picture. I think I am correct in saying that it has a 2.1 bar pressure reducer followed by an 8 bar pressure relief valve. The problem is the pressure relief valve is leaking as evidenced by a steady drip coming through the tundish. I have removed the flexible pipe to confirm it is coming from the cold supply.

I am struggling to find a replacement for the relief valve and as it is designed to be attached to an expansion vessel a like for like replacement doesn't seem like the best option. Can anyone suggest something else I could replace it with?

For instance could I add a check valve and then t off to a pressure relief valve like this one at Screwfix https://www.screwfix.com/p/pressure-relief-valve-x/8131v.

Alternatively would replacing the cartridge in the existing valve be likely to stop the drip? In which case does this item look like the correct part https://www.ebay.co.uk/itm/Reliance-8-Bar-Blue-2116-Pressure-Relief-Cartridge-ZRC214009-/202795519742?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338268676&toolid=10044&customid=CjwKCAiAtdGNBhAmEiwAWxGcUjd--hx3dRRCCCcKxf59i6cW4avPVxe0Y6i-V9Q8TxkQlUCkuR86oRoCu4EQAvD_BwE

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CBW

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Are you G3 registered /Qualified? As you need to be working on unvented cylinders.
 
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Thanks, I didn't realise that I had to be. I'm sure you know the answer by the fact I asked the question.

Does that also extend to replacing the cartridge in the pressure relief valve?
 

CBW

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Yes, vast majority of work on an unvented cylinder system must be performed by someone who is G3 registered/qualified. I had an incline, but you could have been newly qualified asking ;)
 
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Sorry, one more question before I go and call my plumber. Just confirmation that cold water supply to the tank is also considered part of the appliance. So I can work on the water system anywhere except between the first (black) pressure reducing valve and the outlet from the tank. Not that I would be doing anything on the outlet side until it reaches a tap.
 
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