I am assuming the tiles were sealed with wax or something? You may have to strip this back, just on those tiles in the process. Also, a ‘water-mark’ is a bit of a misnomer here; pure water would not leave a mark, the mark is a result of whatever impurities may be in the water. Given that the leak was from a radiator, there is a high likelihood that the mark is from the dark sludge that builds up in radiators or even rust. If the mark/stain is some kind of mineral like rust then you may need an acidic cleaner to stand any chance.

If the stain is not mineral based, then an alkaline cleaner may work. If the stain is deep but ‘mobile’ then it might be possible to use a poultice to ‘pull’ the stains out however, I suspect a mineral stain given the circumstances.

You will have to do some experimenting: First I would try an alkaline cleaner/degreaser detergent, this will cut through some of the wax at least and that may take with it some of the stain. Rinse and dry. If this does nothing except take back the sealer, then you could need an acid.

Try some phosphoric acid based cleaner, (Aqua Mix, or HG for example) dilute as directed and leave it dwell on the floor for a few minutes, then scrub with a pad or scrubbing brush, rinse and dry. If you can get hold of some Extreme by Aqua Mix this may be more effective, but it is in limited supply and only available in US gallon containers.

I would not recommend a brick acid or any cleaner based on hydrochloric acid – as this can be a dangerous chemical to use indoors.

let us know how you get on.

Ian