Nick,
No doubt, you'll work out the relative capital and running costs, and some of that will depend on how long you intend to be there for, but if that is for a long time, then GSHP is a good proof against uncertainty over fuel fluctuations/rises/availability in the future. Once it is in, aside from maintenance and running costs, the source of the energy is free - even with a wood pellet boiler you still have to source the fuel, store it and shovel it in (or fill the loading system). In that respect, GSHP has to be a good way to go.
I helped a mate install the ground piping for a GSHP a year or so ago at a large house. A lot of pipe and a lot of trenching, but it's not rocket science and the depths, lengths etc were all manageable. Depending on the overall length of pipe that you need, the available space and the system design criteria in terms of pipe depth, layout (coiled or flat) and spacing apart, you might be digging a long trench winding back and forth and then backfilling, a long wide tranch and backfilling, or planing a couple of metres off and then replacing the lot, but it is more likely to be one of the fiorst two.
A few random thoughts. Remember that a heat pump also works in reverse, so will offer cooling in the summer as well as heat in the winter. Presumably you are looking at a closed loop, water-source system using plastic pipe as they are the most common? However, depending on how much land you have available and its layout, you might want to consider an open loop system that feeds directly in/out of a pond/lake that you construct (as you are having to dig holes anyway) as the herat transfer is more efficient. The pipes shouldn't leak when installed, but you never know, so it makes sense to have some form of leak detection installed.
Mark
