There's no reason a good plastered wouldn't be able to cover that with a bonding cost and then another plaster cost Alternatively if you use smaller boards I would imagine plasterboard adhesive would hold it as the weight will be spread across smaller sections

They usually come with metal chains don't they? Therefore, if you were able to find a joist in the ceiling above, you can put a hook in to it to hang the chain from.

i think that it depends on whether you are tiling to the same height around the whole room. If you are just doing full height tiling around the shower then stopping at the sloped ceiling will look great. If you want full height tiling around more of the room then you need to look at what else there is in the room and perhaps the logical height of tiling will match the door or window height

Thanks all! looks like I've got a relatively even split of people who prefer the standard "walls colour + ceiling white" Vs an all colour. At the moment I am edging towards all colour - but mainly for the easier reason of no cutting in - I'm getting nervous enough of having to do this on a standard 2.4m wall that I could reach easily, let alone this 3.4m one at various awkward angles!