Next question for this flat refurbishment.

The flat is in Edinburgh - many will recognise the cold-water tank
over the bath configuration of 19th / early 20th century flats. There
is adequate headroom for an over-the-bath shower under this tank - ie
the ceiling at that point is level with the door and therefore
something around 2.5m from the floor.

I'm instructed to tile the whole bathroom - all 10 + ft ceiling height
of it ! (I am getting paid). But what about this ceiling area for the
shower? What material should I use for that ? It's about 1 m square
and currently plaster which seems sound.

Thanks

Rob