F used his keyboard to write :
I'm under pressure from Management to lower our shower rail by around 100mm
but I really don't want to have to drill the tiles and to have to fill the
holes for the existing fitting (as shown at
https://www.screwfix.com/p/smith-loc...-2-pack/9070v).

I cannot really work out how you have used that particular fitting on a
tiled wall, to support a shower head rail. Surely the shower head rail
needed to be vertical, and on a wall that fitting will provide an
horizontal bar?

Am I misunderstanding the question?