Thanks Andy
Yes, this job is now finished. I got round 2 of the problems and compromised on the third - the 1 inch strips of tiles.
In the end, after I had levelled out the RHS wall after tiling the back wall 😳I found that the tiles on the RHS of the back wall butted up quite neatly into the corner. That left the strip of tiles to the left, which the customer really wasn't bothered about. In actual fact the LHS of the wall is boxed out by about 100mm plus and it looks like the tiles are centralised to the room, but not the back wall, if that makes sense. So, asthetically it isn't so bad as it could be.
With regard to the 6mm kick out at the bottom. The hardibacker idea is a very good one, and I had used exactly that approach in another corner where I was going onto studwork (side of the 100mm boxing in). I used strips of 6mm hardibacker screwed in - I had them left over from another job, and filled in any gaps with tile adhesive. What made it easier here was that I was tiling all the way to the ceiling. On the wall in question I was only tiling half way up. But I filled and levelled it with rapid set and it worked a treat. I got it pretty level, which made the tiling a lot easier.
Thirdly, the towel rail. That was an absolute doddle!
The approach was to position the towel rail vertical and measure to the wall. The wall in question sloped out a bit (but on the flat) so I measured and cut the brackets using an adjustable pipeslice. I then redrilled the screw holes through both the outer and inner sleeve after having set them in position. It was almost as quick as fitting it normally. In time I'll post some pictures of how I did it on the open forum.
Thanks again for all your input guys.