Flagstones waterproof jointing

[109-1616496369]

I have a paved balcony over part of a garage. It was relaid and a new balustrade fitted but the builder (now defunct) persuaded me that a coat or two of Cromapol would work as well as a membrane at keeping water from penetrating to the garage below. Needless to say it did not. Despite re-grouting water still penetrates through the joints. I have tried a siloxane coating (Enviroseal) with limited effect. What I need is a flexible waterproof jointing compound such as expansion joint mastic, but I am reluctant to choose this route without some expert guidance. Relaying the flags over a membrane would, of course, be the best answer but this is impractical unfortunately. Thanks in anticipation.