Advice Needed - Please be gentle!

[Bill]

Would that not damage the tray underneath?

it shouldn't as you have already stated that 300mm around the drain has been redone

 

[John Benton]

Tom could be right about tray. From the pics there doesn't look to be any fall on the tray at all. There should be a specific gradient on the tray that the mosaics follow so as the water naturally runs towards the drain as in the pic below

 

[SJPurdy]

watching the video clip with the shower on gives me the impression that there is more wrong here than just the tile thickness not being sufficient to overflow into the drain. The water seems to be making a large puddle and not flowing towards the drain as if there is no slope towards the drain! Are you sure the shower base former is fitted the correct way up? or have the tiles around the drain been built up so much that the slope has been almost totally cancelled out?
I can't see a simple solution with the information given but I think it probably needs at least the tiling redoing with a tile thickness that is compatible with the drain thickness.

 

[White Room]

Probably been better off using something like these.