Just an update to complete the thread.
This system was comissioned yesterday. Yes it's a slow build being done as a background job by the main contractor.
The wiring of the system was really quite simple. All the GSHP needed was a power supply, and connection to a number of temperature sensors, two in the hot water storage tank and one outside. The two tank sensors were easy as they used some of the "spare" cables I had pulled in at first fix. The outside sensor was harder. I wish I had know about that at first fix. It was supposed to be on the north facing wall, but had to settle for it on the west wall instead.
The GSHP has all it's own controls and basically runs 24/7 to maintain the heat store at temperature. the UFH runs independantly just drawing hot water from the heat store as required.
The supply rating was 40A but it never needs that much. Most of the time it just uses the compressor to extract heat from the ground, but once every fortnight it uses the immersion heater to heat all the water above 65 degrees to kill any legionalla bacterea. The immersion heater for that is rated at 6KW.
And the only other control was a sensor and valve in the pipework that's an over temperature shut off should the tank overheat.