Well the overnight brain cells did the trick. Roger came up with a solution. He used a section of tube, with a slot cut in it, and a groove at one end to take the piston rings in. Once this is placed in the chuck of the lathe, as seen in the photo above, and the chuck is tightened up, this squeezes the tube enough to grip the piston ring whilst in a fully compressed state. This then allowed the required thickness the be removed from the inner edge of the ring. Excellent. You can see the slot in the tube running diagonally across the lower side.
As of lunchtime today (I am typing this in as I eat, hahahaha) the 1st piston is fully installed with all the piston rings fitting inside the liners, and the big end connected. The 2nd piston is in place in the liner ready for the piston rings to be fitted.
Unfortunately after lunch we discovered that the oil rings are not the right ones, so we have had to order some different ones, which should take about a week to arrive.