.. and then stare he is going to remove the shower tray and redo it.
Well, if the shower tray is going to be redone, someone's going to have to do it. I certainly don't want to get the previous plumber to do it (and in any case, that ship has sailed now, as I've sacked him), and the new plumber would understandably claim zero liability if the tray cracks on removal, so I don't see what I have to lose by trying to carefully remove it myself?
And if setting it down on a sand & cement base is more complicated that I currently think it should be, by all means I can pay the new plumber to do it. But if I did it, I would follow the manufacturer's instructions regarding sand to cement ratio etc. and would probably do a much better job than our cowboy plumber did with his tube of silicone. I would also use the provided template to cut out the hole for the waste, but would leave connecting it all up to a qualified plumber.
I'm getting back to my original thoughts that he is doing the work himself.
Here we go again... If that were the case, why wouldn't I have just come on here and said I'm doing some DIY plumbing? It's not a crime is it? I don't need to hide behind an elaborate story where I'm pretending to be a plumber!