Here is the long story. I inherited a system with a F&E tank, which was noisy. We converted the system to a sealed one. But it was not properly cleaned. In the mean time the HWC failed as well. My trusted heating engineer (not involved in any of this previously) cleaned the system chemically and flushed (both chemically and with a pump) the boiler heat exchanger as was very sludged due to previous owner neglect and following sealing of the system. We discovered that the boiler was overheating and was not going over 68C (locking out). The location of the magna clean filter was initially wrong and that was rectified. Currently the system is clean, the boiler finally dropped gas consumption, the plug you suggested had a further positive impact. It is nice and warm at home now. The system is properly balanced with temp clamps. The boiler is properly condensing. In fact the radiators are not hot to touch. Which is different to how they used to be, and still it is much warmer and nicer temperature wise at home.