Goat Eyes
Striker
If the gap is pretty big, try and fill it first then seal it.That's why I'm going to regrout and silicone. In my original post I've seen that the grout I have linked in seem to be the best grout. But previously its been the silicone 3 times. Previously I've taken the plastic form off from round the tray and looked where the water comes down and it's been when I've aimed the shower at the join between the silicone, tray and tiles. I'll still regrout as then I can also rule out what you mentioned.