Should I fit the shower tray or the panelling first?

The shower tray. Install the shower tray in accordance with the manufacturers instructions to ensure it is totally rigid (there needs to be zero movement when you stand on the tray). Completely seal the tray to the wall(s) so that no water can get down the joint, using silicone sealant. Next fit the waterproof wall panelling down onto the top of the tray surface, and apply a second bead of silicone along the joint between the panelling and the shower tray. If you are fitting the shower cubicle on top of the wall panelling make sure that there is enough adjustment in the frames to allow for the extra thickness of the panels.