[Graham]

I have a couple of ground clearance blocks but recently been thinking of something to actually measure it with.  How about a digital tyre tread depth gauge fixed under a surface plate, something like this that's only 4 quid posted.

I think that is too complicated and only gives a point measurement, the metal washer is very quick to use and covers a reasonable area of the chassis (mine is 25mm diameter). if there is a gap between the car supported on the washer and the car tyres not touching the test plate the car fails the test. I have used the Scaleauto step guage previously, but actually it is less accurate than the washer (1.5mm step is actually 1.49mm) my washer is exactly 1.5mm, and also the step guage is difficult to get a consistent result. 

Mind you I had to search though a box of washers to find one of the right thickness