Hmm, so I now have another problem.

I set laser on the milling machine head, just needs to be attached to something solid, then levelled the laser to the machine frame which is sat on a sturdy bench. The problem is that walking near the laser causes microscopic deflections in the concrete floor, sufficiently large to show up as 10-20um of error 3m away on the machine frame. These are probably amplified by the height of the laser from the floor, about 1.5m as the same doesn't occur walking near the bench.

I think I will need to move everything directly onto the concrete to stand any hope of getting things stable enough to do any frame levelling. Problem will then be not kicking it during the process.