The Ground Source Heat Pump pipework arrived today and the Rainwater Harvester pit was completed. The harvester was in the ground but by Sunday evening the pit had refilled with water and the tank - still empty and so extremely buoyant - has floated to the surface.
We can't tell if any damage has been done until the ground workers arrive tomorrow morning and move the digger which is currently pinning one end of the tank down.
Fingers crossed its okay.