The message .com
from John Edgar contains these words:

Thanks for all advice. According to Mira we need a new "cassette" in
the shower at a cost of some £130, plus fitting.

Why not fit it yourself?

Try the Shower Doctor (showerdoc.co.uk) for spare parts. You may find
you only need a particular component or an overhaul kit. (Whether they
do an overhaul kit for this shower is something you will need to check).

--
Roger Chapman