The next thing to consider is the floor it's mounted on. The workshop floor is thin plywood with the thicker flooring under it and 2x1 bearers under that, all on top of a 6" thick concrete bed. My Dad understandably doesn't want to cut a hole in the floor to mount it on the concrete.

Could we put the sheets of 18mm plywood used for shifting it underneath (how big, ideally not the full 8x4) to spread the load. Would that be stable enough?