water under my floor tiles in downstairs cloakroom

This sounds like a leak somewhere, which then let’s the water go under the tiles, unfortunately the best way to solve this is to remove the tiles & find out where the leak is

One solution would be to use a coloured dye on toilet, and basin, these dye's can be obtained from a plumbers merchant, so by process of elimination, use this when flusing the toilet, and basin waste, but area must be as dry as possible first, if no coloured water from the dye is noticeable from this exercise, then it could possibly be pipework under floor leaking.