Heat-pump vs condensing boiler

for heating, though they can be run in reverse.

I have Underfloor heating and I did consider a Heat Pump ..... but Timber Frame manufacturer advised of well known Heat Pump based system was in one of his house ..... the occupants were freezing, heat pump could not get house warm. Solution was a Propane powered space heater .... blasting in hot air, once house was warm then heat pump could keep it that way.

I decided not to use a Heat Pump based system

I'll wait maybe Combined Heat & Power units are the way forward.

implying the pump and/or ground array were undersized* ... what did the well known manufacturer have to say about that?

I believe the *slinky* pipe system can suffer from dry soils leading to poor thermal transfer. If my house was closer, I'd be tempted to quietly put a thermal collector in the river:-) Nice and warm output from Luton sewage treatment and much warmer than subsurface water.

Not like Paul...:^)