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The manifold should be a complete with pump and integral bypass look on line and you will see what it should look like I would be surprised if has ever worked properly. https://bhl.co.uk/uponor-7-port-v5-compact-control-pack-1059848-wp88512.html
The flush machine has the pump that’s why I said turn off the flow and return and connect the machine the magnet unit is great it speeds up the job, I used it in conjunction with the Kamco machine.
If it’s flowing through too fast water won’t get hot enough, having to slow the water flow points to that you may need to get a pressure reducing valve ion the cold supply to the combi boiler. You would be better with a heating engineer than a plumber to diagnose
Pretty sure there should be an insert for that. . Washing Machine Fabric Conditioner Cover J00516811 - Hotpoint - Hotpoint - https://www.hotpoint.co.uk/washing-machine-fabric-conditioner-cover-j00516811/p