Stuart wrote:

Sorry if I sound thick but isn't it the panel that needs trimming
...I'm just thinking of how my bath panel fits in a groove under the
lip of the bath .


Probably didn't put it too well!

Think of the side panel as having the top and bottom bent over at 90deg
to the vertical, like a square bracket sign : [ , with these upper and
lower "flanges" about 4 inches.
This slips in under the lip of the bath.