With a ground source heat pump, you're picking up the stored warmth held in the ground that's a fairly similar temperature all year round, but as Air Source heat pumps are trying to extract heat from a rising and falling outside air temperature, the coefficient of efficiency is undeterminable. Okay for autumn, crap for winter. The other benefit of a ground source pump, is that you can reverse some systems in the summer, and get air conditioning.
I'd love a GSHP Doggit, but I'm told I need a garden with a footprint of treble the square meterage of the house - which I haven't. Bore hole alternative method will exceed my budget for CH.
Ideally I'd have as green-a-set up as poss - with an ASHP being powered by PV, with battery storage etc etc, and solar thermal working with my woodburner in a triple coil cylinder (!) but I've just not been convinced with the figures and payback of solar. Shame.
I'm hoping the ASHP will take care of March-Nov demands, and the woodburners' back boiler will go the extra mile for the winter.
I will get a installed quote on the ASHP with the RHI payback - if it's around £7500 installed and I can get half-ish back over the 7 years then i think it's worth a pop- I've kept my oil tank as a precaution if I need to pop a boiler back in.