Well, I just started pulling them up and although one of them has come up without much trouble - and still pretty wet underneath, the rest are hard enough round the edges that I'm struggling to even get anything in to try and prise with, so I think I'm going to have to leave them there to go off slowly in their own time - it's only a kitchen, so not high traffic and there's no damp.
Any opinions on when I'll be safe to grout these? Am I trying to do it when the stuff has dried 100% right to the middle of the tile (if so, how would I know) or is it when they are solidly set in the bits I can reach?