I am faced with having to get a custom shower enclosure made up to fit a
fairly unusual installation under a sloping roof.

Can anyone recommend where to get the FITMENTS online? - local glass
suppliers are happy to cut and drill the glass and toughen it, but want
huge and silly prices for fitments.

I would say that channels rather than clips are preferred - each piece
of glass will be supported on tow or even three sides (inc shower base)
apart from the door which is hinged off one of these..

Surface to be mounted to will be either the shower base, or a tiled
wall, or plasterboard ceiling..

Oh, and does one leave a gap at the top to let the steam out? Easiest
place would be at the door - simply leave it a couple of inches below
the ceiling..