Thanks Markus, that's hugely useful. I did manage to get the number of 892 from a broadside photo of the Piko model. Their check digit is wrong. They have -0, it should be -5. I've also run the calculation on these numbers here:
Unless I've used the wrong formula (I don't think so, I've seen it mentioned in several places), the numbers should be:

827-1
707-6
891-7
871-7

The model companies just seem to pick a number at random and hope no one notices.

Alan