I have a 4mm gap between my shower tray and the tiled wall, I have tried just sealant but its not working as my wife is quite heavy and when she gets in the shower tray pulls at the sealant and the wall tiles and gaps appear. Is there something I can fix along the tiled wall that will overlap the shower tray lip but still be flexable and remain sealed? Thanks

Unfortunately the shower pan appears to be not adequately supported. How big does the gap get when your wife gets in? Might consider caulking when the gap is largest. That was an old tub trick, fill it with water and then caulk the edges.

Your shower should have bedded at the time of installation. The lack of support under the shower floor is your problem. Is there any way that you can get under the shower floor and either build up cement or wood to give the shower floor. Some support? Let me know. Back to you, Tom