walk away, if you cant fix it down properly using the manufacturers reccomended methology, ie screws and plugs, and u just silicone it, your responsible when a large ar*e sits down , the pan breaks free and cracks and leaves the current occupant needing 50 plus stiches. My working ethos is, if you cant do it properly dont do it at all, silicone is used far to much for botching jobs imho, use it for sealing baths and basins along the tile line and nothing else, thats what proffessionals are supposed to do. Hence why the underfloor heating should be laid correctly first time round, get the customer to call back the original installer, if its electic uf heating have they also laid it under the bath feet? great way to cause it to fail early.