Gap under shower tray

Hi All,

Frequent reader, first time poster.

About a year ago installed a stone resin shower tray as part of a whole house refurb. 18mm ply screwed to joists, mortar bed, tray laid. All went fine and shower has since been tiled. However during the last year something has 'moved'. Its unclear wether its mortar shrinking, joists flexing under load of the rest of the bathroom, (including self leveling compoind on the floor), or something else settling, however there is now a small amount of movement if you step on the end of the tray.

I've cut out the floor at the end so could investigate, effectivley there is 0.5 - 1 mm gaps between both the tray/ mortar, and mortar/ ply that close up when the try is loaded. I've been able to get some packers between the ply/mortar, which has helped, but these are obviously only supporting the edge of the tray.

Options:

1: do nothing/tolerate small amount of remaining movement. Concern is eventually tray might crack, although movement is fairly tiny and should be further stabilised once shower screen sits on it. Am I right to be worried?

2: rip out tray. This is more expense and faff than I'm really willing to go for, as would involve removing the tiles and possibly deconstructing other parts of bathroom (eg wall hung toilet frame and boxing in). Also not sure its possible to lay a new tray in that alcove with taking the whole room back to first fix stage (this one was lowered into position via suction cups and a pully on the joists above.

3. try to fill the gap with something? The only thing a collection of minds can so far come up with is self levelling compound, and either build a moat so it 'runs' in, or inject it in directly with a syringe! Clearly it wont bond to anything as nothing is primed in there, but will fill a gap with something non-compressible?

Any thoughts?

Thanks in advance 

Ben