Am planning on putting in a new floor in my bathroom using either real
stone slate or porcelain tiles. The floorboards are quite eneven so as
I understand, I need to put down something else first.

Diydoctor.org recommend using 22mm plywood on 'quarry' tiles. Surely
they can't expect someone to raise their floor by 22mm? It says 6mm
plywood is fine for vinyl tiles, and a friend also said it was ok to
use 6mm.

Can anybody offer any advice? Anything else I should watch out for?

Thanks, Miguel