How old is your flat? I was told a different story, they encourage me to overlay kitchen and toilet tiles. They claim that as long as there is no hollow tiles or damaged tiles, they'll just do water proofing before the overlay. So from what I know it's like got 2 layers of water proofing if you overlay (1 old, 1 new). The above statement is my 2 cents, I'm a green horn afterall so experts please advise.