I currently have a 800x800 shower tray in an alcove which is bigger,
and it has been installed with tiles bridging the gap that = crap seal;
and a sod to keep clean!

The alcove is 895 wide (wall-to-wall - e.g. with tiles removed).

Ideally, I want a 900 wide shower tray. But the only "decent" one I've
found is 900 wide "inside" and 920 wide "outside".

I'm happy to dig out 20mm out of the walls to get it in - but obviously
need to see internal dimensions to ensure it is possible.

But before I plan in detaul - any better ideas!?