In my humble opinion (I am not a professional) the shower screen should be fitted over the tiles and not on the wall alongside the tiles. That's why it's so important to ensure the tiles are hung square and plum. The reason I'm responding to this thread is because a family member had a similar situation and he was constantly having to reapply Silicon sealing on the inside of the shower screen to keep it all waterproof. We eventually replaced the cubicle. How has the fitter fitted the shower tray also? Did he/she tile down to the top of the tray, or did they fit the tray after completing the tiling as was the case I'm describing here and which also created a number of issues for my family member?