Mh70 wrote:

Not working display reads f28

Isn't a glow-worm climapro2 RF a wireless thermostat, rather than a
boiler? The manual is easily found online, it seems it has human
readable error messages for the obvious things like "low battery",
"communication error" or "pressure too low", so probably not those.

Does this error appear on the remote handset, or at the boiler end?