The supplier of my quandrant shower tray suggests bedding it on sand and cement even when mounting on a wooded platform. This platform will be sitting on the existing bathroom floor boards. What is the purpose of the mortar bed? The platform is made of 15mm ply screwed to 4"x2" bearers. The mortar will not adhere to the wood anyway.
Many suppliers suggest bedding on sand and cement to get the tray level Julian, but you are quite right, it is totally impractical on a timber base. The tray can be bedded very sucessfully on a bed of Gripfil adhesive which will not only help to level it up but make sure it never moves again! Go to the Diydoctor project on Adhesives used at home to buy a tube of gripfil and an applicator gun.