Thanks for the suggestions. Joe's idea with the drill bit is a good one, and I'll probably use it - but the real problem is determining the thread, and an ordinary thread gauge is no good for doing that with a hole. And there don't seem to be any similar screws or bolts on the lawnmower.

One idea I've thought of would be to screw into the hole a rod made of some substance that would take an impression, so then I could unscrew it and measure the thread. Maybe wood or plastic...

.............

Officially just about everything (aside from time, pints of beer, and speed limits on the road) is supposed to be supposed to be metric in the UK these days. But in practice both systems co-exist. I suppose as the older generations die off the old measurements will as well - though it might take longer than they think. For example I've never heard young people talking about their weight use anything but pounds and stones.

Next thing up, I suppose, is trying to make us drive on the wrong side of the road.