Hi Rod.
I had a cracked shower tray in my Abbey 540Se (swift group product).
I had to take my tray out for repair as it was cracked between the ribs on the floor of the tray. The cracks (2), were about 6-8 inches long.
To avoid having to take the toilet out to get the tray out in one piece, I made a cut with a dremil type tool along the small lip between the top of the tray and the loo base part of the tray. This lip is covered by the shower door when it is in use, so unlikely to suffer from leakage. It does take a few hours to extract the tray carefully avoiding further damage.
The root cause of my cracks was the foam board packing piece that is supposed to support the tray base. Having removed that from the tray I then claened the underside of the tray and key'd the surface with coarse sand paper. Now using a glass fibre kit from halfords, I glassed over there repair using three layers of glass mat over the crack and covering about 2 inches beyond the cracks all round for support.
The after the resin ahd set (overnight), I used 3 tubes of bathroom sealant to fill all the undulations on the underside of the tray prior to putting the foam pad back, using the sealant to bond the foam to the tray.
Then it was a simple task ( another couple of hours) to re fit the tray.
Finally I sealed the cut I had made with a bead of sealant that converted the step from the tray to the loo stand (10mm) into a 45deg slope to allow any water to run off quickly and not hang around the cut I had made.
This repair was made about four months ang and has stood the test of time without any sign of failure, despite us living in our van full time. And it is still virtually invisable, you would need to know where the repair is to spot it.
One tip I would offer is to put masking tape on the cracks on the smooth face (the side you see when fitted) to reduce any rezin bleeding throught the cracks.
Sounds complicated but so long as you are a competent DIYer and have a spare weekend it can be done. Saving you