Has to be air Rik, I rent the unit so wouldn't want the expense of installing a ground loop only to find a year or so latter the landlord wasn't renewing the lease. He has provisional planning permission to build houses but he's not in any rush as he has lots of better sites to build on. Doubt if it will happen anytime soon to be honest but still not prepared to take the chance.
Think there are lots of Solar Companies looking at this sort of thing, it's another way of saving money long term so fits in with what they do to an extent.