john kirby laid this down on his screen :
>> Meanwhile the Grundfos pump is spinning day and night even with the
>> boiler switched off.
>> Grateful for any insight as to what the problem might be.
>
> If it is a certain model of Grunfos pump, a UPS2 - then yes it did cause
> problems with the relays on boiler PCB's and cause them to weld up with
> the surge. This item is supposed to fix the problem...
>
>formatting link>
It may prevent the problem from occurring - but it ain't going to unweld the contacts in the boiler if the problem has already happened.
Anyway, that's very unlikely to be the problem. ISTR that the OP said that the pump was running even when the boiler was switched off. If that is the case, the boiler can't be controlling the pump. Chances are they're wired in parallel.
The overwhelming likelihood is a zone valve stuck open an/or it's micro-switch contacts being stuck or welded shut.