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.


The Heating Engineer has proposed a new boiler in view of the problem and
its age.

Stangely - despite his website saying he fits Worcester-Bosch and
Veissemann, he is proposing a BAXI Ecoblue

I guess based on ease of fit. Are they any good?