neil wrote:At £6k a borehole it would work, at £30k

I think I'd be looking at a new gas boiler. Will get some quotes if we get the place.
Neil, don't forget you get the RHI for GSHP which, depending on your house heat loss would certainly pay for most if not all of the install. In our case it covers the install and a few years of our bills.
Another update as work has been underway for a week now. Kevin from BIR has been here digging in the 2 x 400m loops round the field. Installing the collector and running the pipes back to the garage where the heat pump will be installed.

4 trenches round the edge of the field.

Last bits to be filled in.

Keven setting up the fusion welder

Welding a joint