From browsing the tool shops, it looked like they weren't readily available so last time I was at Kempton Park bike jumble, I bought some 22mm loose sockets and a length of 22mm Hex stainless bar.
A bit of time on the lathe and some MIG brazing later, I have 4 22mm Hex Keys.

If any of you need one, let me know. I'll happily stick one in the post.
Mark