Well a RESULT
Meeting this afternoon with 'smooth the customers feathers person' from both SE Water and Clancy Docwra went extremely well. They have agreed to all my landscaping, pressure washing, fencing and replanting requests, have agreed to compensate the chap who cut he hedge for them (with a suitable bottle of spirits!), and offered to pay for the use of my digger (which I declined).
Then they agreed to sort out the high pressure either by reducing the main pressure or fitting me a PRV. Then they spotted a technicians van in the next door pumping station, and went to collar him for advice on reducing the pressure ....
.... he was working on a PRV on their distribution system THAT WAS STUCK FULLY OPEN
They have agreed to fix any of my pipework that is still leaking after their PRV is fixed, and they must now have fixed it as I've just measured the pressure as bang on 5 bar
Taken the dogs on a tour of inspection of all the fields water distribution systems, and the only leak left was a 25 mm MDPE elbow on a cattle trough where the pipe had been pushed out of the seal (presumably by that intense water hammer that cracked the second main. A quick (well not SO quick!) trip back to the barn for some spanners and that was soon fixed.