Ok, There is one area of the bathroom grout(one tile) that has cracked.
I don't want to lift it at the moment, as SWIMBO is thinking that she
would like it all changed!

The tile moves slightly when I stand on it, so this is what has caused
the grout to crack.

Can anyone recommend some sort of flexible grout, or is it likely I
could squirt some thing into the gap that the grout was in, that would
run under the tile and stop it moving.

Its only for as long as I can put her off replacing whole bathroom
floor, the nicer I can get it, the longer I can go without replacing it.
--
yendor