On Saturday, 3 November 2018 17:50:36 UTC, DerbyBorn wrote:
After lots of checking a Heating Engineer has decided a mate's heating
system has a partly blocked boiler heat exchanger. Likely? 12 - 15 years
old. Has been pressure cleaned a couple of years ago.

Unlikely. But "silt" (metal oxides" could have wafted in from elsewhere in the system. Should be easy enough to dislodge if the boiler was taken outside and the garden hose applied to the connections.

The radiators could need the same treatment.
If there is a lot of crud inside, it points to corrosion problems in the system, the radiators may all be shagged.

Benefits of anticorrosion treatment in the system.
Made worse if the system has been "pumping over.