My query was about the position I should treat as the centre of the wall
rather than how to arrange the tiles on the wall once I have determined
the centre (I've done any amount of tiling over the years but have not
run into this scenario previously).

As described previously, the wall is not the same length throughout its
height along the length of the bath. It's shorter just above the bath
and just below the ceiling, but longer over the height of the window
where it runs into the reveal.

A spot of ASCII art might help.

----------------------------------------------****
| | *
| | *
| | * Window
| | *
| | *
| Plan | *
| | *
| |****
| |
| |
| |
| |
| |
----------------------------------------------

The room at the ceiling and floor is a rectangle as shown by the | and -.

At one end is a window which is not centred across the wall and which
means that a plan drawn at the window's height off the floor will be
extended as shown by the *.

So, on the wall at the top of the diagram, where is the centre to work
off for tiling? Is it half way along the
---------------------------------------------- section only (ie half the
length of the room at floor and ceiling height), or is it half the
length of the wall at window height (ie half of the total length of
---------------------------------------------- and ****)?

TIA

--
Frank
(Beware of spam trap - remove the negative)