Thanks for the reply.
Using good quality discs (as far as I know), at x24. Burning on a mac.

But I have also used discs supplied by this Euro Sat Navs company, who claim to have successful upgrades.

I've asked 3 local BMW dealers about upgrading the OS, but they look at me like I'm mad and just show me map discs, so I've not been able to get hold of BMW originals to try...