While I have no direct experience of ground source heat pumps, my sister investigated thoroughly and decided not to go ahead. Their problem was that the radiators were not big enough to cope with the lower heat of the hot water, compared to a boiler. Yours seem to be running at a low temperature and may well be large enough. Idealy GSHPs work best with underfloor heating as this runs at a lower temperature than radiators. Do you have sufficient height to lose 100cm downstairs with 50mm insulation and 50mm screed containing the pipes on your floor? Other considerations are - do you have enough land to dig the trenches for the pipes or do you need the more expensive borehole? While not a diy task, external insulation has many advantages, you do not lose room size and the thermal mass of the stone / concrete serve to stabilize temperatures inside.
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Simonridout