Thanks for the answers. But I'm none the wiser.

Could it be that each Water Co. does it differently?

I am sure that ours has a device that latches when turning
off and slips when turning back on.

I had a v good look at it having un-screwed the head.

I found that the head which you turn has an underside with
lugs that are a bit like those metal screws that you can't
unscrew for good reason.

Wedge shaped lugs engage the lower part of the tap,
90 degs one way (off) and a long slope the other that unscrews what
looks like a wood-screw holding the head down so that it won't engage
and open the valve.

So, any more ideas?

N