Just to note that replacing two right-angle 38mm plastic joints at the top of one of the boreholes seems to have fixed this. Two months later there is no, or insignificant, leakage. These underground joints were removed and replaced once with no improvement, but on replacing with new ones all seems well. I cannot see anything wrong with the old examples, even after careful cleaning, but I suppose something must have been!
The deductions already made as to which loop (that is pair of boreholes) and which borehole (return-side hole because of the no-leakage reaction to continuous pumping) were both correct. And the green flourescin dye very helpfully confirmed the leakage site, though the amount of dye already in the ground on digging resulted in a green pool which made the exact source of the leak hard to discern.
The sub-contracted plumber has sent the builder a bill, and the builder has digging costs; totalling a few £100, but we have firmly said this is not our problem!
The ground temperature (that is the circulating ground loop temperature with no GSHP working) seems about the same as last year at about 10C - so groundwater movements may be enough to stop it cooling down indefinitely.
The system is otherwise fine and works well. The similar system installed in our new village hall (with underfloor pipework) also works well - but it has taken us 3 re-submissions, a number of phone calls and £1k worth of heat meter fitted inside the GSHP casing to reach 'level 3' of the RHI (government subsidy) system. It appears that we are nearly there but we have 'lost' the last winter. If all goes well the subsidy should about cover the electricity running costs. We hope the much-postponed domestic subsidy system will prove simpler....
