Finally got round to emptying the store of sheet materials from the garage en route to the shed.
A friend is looking for one piece, and so I've been measuring up. Nominal 18 mm comes up nearer to 20 mm on the digital callipers. I did a rough cross check by setting the calliper to 18 mm and comparing it to a measure, and it does seem to be more or less 18 mm so the board is probably thicker.
I am now wondering if the boards are manufactured slightly over size, or more likely they have swelled a little after having been stored in a cold garage for many years.
Cheers
Dave R