@Radar thanks for the detailed reply, apologies I only just saw this.
So far I got the sheet, bearing in mind I’m doing only Europe 1940 edition for now. I trialed some ferite magnets with a 0.6kg pull, but they were not enough to reliably attract through the board, then I bought a sample of these and they are pretty strong. That’s about as cheap as I can find for that strength.
I should probably try to find a print out of the game because with 400 pieces these magnets will cost around £200!