I'm glad someone checked that - I'd been working in metric and thinking metric. It wasn't 25mm at all - it was an inch, which for a quick measurement of say three or four inches is accurate enough for many purposes especially when you know which side the error is on.
Went to Lloyds bank today to get new £1's. Told that haven't got any and would only get them in if ordered by them when running out of old £1's. I suppose they know what they are doing?