Thanks both. I'd rather not use mortar - seems too inflexible for my liking. I'd have gone with a Mira tray if I could have found one where I didn't need to notch-out the joist and move pipework.
My feeling exactly. I rather like the idea of using expanding foam as advocated by this fella:
However, even though he claims to have fitted over 30 without any problems, I don't think I'm brave enough.
Quick update - SimpleBathrooms, aka Homefast Bathrooms Ltd who appear to be a sister company of Aica, have replied saying the tray should be fitted to a bed of mortar. Makes sense, but not sure I'll be buying another cheap tray from eBay.
I've recently bought an Aica 1400x760 cast stone shower tray on eBay from SimpleBathrooms - https://www.ebay.co.uk/itm/303699029436
The fitting instructions are pretty much non existent with no metion of what to bed it on. I contacted Aica who are insistent that nothing should go between the...