Glow Worm Flexicom HX boiler F11, F22, F25 Error codes
The Solution which DID work

When our landlord let us have the heating engineers come round to have a proper look, the engineers discovered the system was clogged with rust and muck. They said it was the worse they had ever seen. The house is as around 20-25 years old, and they believed the system had never been flushed. So the heating engineers did the following:


Flushed the system
Used tools to vibrate all of the crud out of the radiators and flushed periodically
Fitted some kind of filter/ioniser which will attract all the particles into the filter and this can be flushed at every yearly service 


Outcome

We now have all radiators really hot. The house is pleasant to live in. I have not even heard the central heating pump running since then, as I guess it needs to run less hard and less often to get the flow through. Tap water is really hot again, so have been able to turn boiler down to 70°C. I may be able to turn it down further over the next few weeks too.

Hopefully some information within this post will help someone who experiences similar issues with their heating system or boiler.