Is there any possibility of moving the shower tray closer into the wall? Its just that any gaps like that often harbour problems for the future.
Do you intend to tile the brickwork that we can see?
It has been previously suggested to me to under cut the wall so it hangs over the tray, put in shower paneling to build out the wall, or put in window trim around the gap.
The two walls are not exactly at 90 degrees, so who ever fitted it, didn't fit it properly.
I've decided to go down the option of putting window trim around it, as that is cheapest/least effort option. It's in a guest bedroom so it only gets used ocationally. I just don't know what is the best filler to put under the trim.
You could try maybe a upvc angle strip siliconed into position but that will fail eventually as will every other 'solution' apart from refitting or replacing the tray closer to the wall. Needless to say absolutely meticulous cleaning of the existing areas would be necessary.
If you really must......
Rake out the crap between the tray and the wall, and then trowel some quick set sand / cement mix down the gap. Allow it to set, slightly undercut it and apply a layer of silicone between the tray, the cement and the wall.
Any tiling / wall boards or whatever need to sit on the edge of the tray, bedded down in silicone too......wall boards would fill the gap much better than tiles!
I could under cut the walls so the tray sits under the tiles, but I don't know how to do that, if someone could explain how I could do it I might attempt it, as the general consensus is that is the best long term solution.
The theory would be to cut the tiles at tray level, then cut back the plaster underneath with a bolster chisel to allow the tray to be tucked under.
However, there may be an issue with the cubicle frame not fitting afterwards, so try to consider that too.