Wet tray and insulation under

[average]

have to install a new wet tray in my bathroom conversion. The rest of the house the floors have been finished with dpc Radon barrier, 3 inch concrete, 3 inch insulation, heavy gauge dpc then 3 inch of screed. In my bathroom, I haven't installed the insulation or screed yet! Will be doing this at the wknd however, I'm wanting to install a kaskade wet tray and anything I see online about installing them onto concrete, it's straight onto concrete subfloor with no heavy gauge dpc. It's showing insulation glued and laid onto concrete. I'm thinking if I'm to install mine like this, the tray will move because I have heavy gauge dpc sheeting laid on top of concrete. Will I be correct in thinking, ill have to lay more concrete instead of insulation in this area for the tray to sit on rather than have the insulation glued to sheeting.