Help bedding in a shower tray

I have just bought a resin stone shower tray (very heavy) and was wondering how I should go about bedding it in. So far I have complied with the manufactures instructions (3/4" exterior ply screwed directly to the joists and plaster cut back to brick around the tray so that the tiles overhang the tray).
To accurately level the tray I am considering setting small blobs of mortar mix in the corners and adjusting them to the correct height for a level tray, then filling the remaining area with mortar slightly above the blobs and pressing the tray onto it until it presses firmly onto the set blobs giving me a level tray. Does anyone see any problems with this method or know of a better one. Lifting the tray into position is another problem to overcome.
Also the manufacture suggests using a sand cement mix, silly question but I assume this should be a wet mix not dry. I've read postings in this news group suggesting silicon as a screed, is silicon not that white flexible stuff for sealing baths (i.e. too flexible it would have no affect in supporting the tray and the tray would wobble about.) Please help and convince me that the silicon referred to is not the stuff I think it is.
| |<-Plaster Wall | | | | | _| <-Brick Wall ============================| Tray | ____________________________|__ _____3/4"_Ply__________________
Thanks in advance Martin.
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The pukka way to do this is to bed it down on a 5mm ish layer of 4:1 semidry mix.
This gives full support, and unlike Silicone will not invalidate your warranty. I have recently installed some Just-Trays shower trays (resin stone cast) and they specifically state not to use Silicone.
Rick
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