Hi,
I’ve just called Microsoft support, and they were totally unhelpful.
Apparently Microsoft don’t make replacement keys available and I was advised to request a £200 exchange for a refurbished type cover or buy a new one for £150.
I pointed out this was completely unreasonable for such a delicate device that is bound to get damaged on occasion, that replacement keys for previous generations were available, and asked where I could complain about the situation. I was given a shortcut URL address which pointed me to a European Union website!
I’ll never be buying anything from Microsoft again.