Christian McArdle wrote:
I could fit a long thin strip of WBP ply over the gap along the
length of the back wall so that it over hangs across the tray by
half an inch or so, then I could seal / stick it to the walls / the
shower tray with no more nails or silicone or something where the
ply meets the tray, then tile over the ply and seal with grout.

Should be fine. Plan the tiling so that you don't rely on
sealant/grout for the majority of the sealing. Basically put the
"horizontal" tiles down first and then tile down the wall to them, so
that water running down the wall hits the horizontal surface.
Obviously, you want a good fall to the horizontal surface. 45 degrees
is good. ;-)

Also, I'd use some screws in there. I'd cut some triangular brackets
from some pine, screw these to the wall and screw the WBP to these.

Christian.

Thanks Christian

Sorry, I should have stated that the shower subicle is already installed and
has tiles in place already. The problem is that whom ever installed it
didn't do a great job with their tape measure so I'm left with this gap to
re-seal when in fails every year. If I put some WBP ply in there it will
have tiles to the left, right and rear and putty / shower tray below it so
I'm not sure if it's a good idea or how to secure it in place.