In message , Pete C
writes
There might be a strainer on the inlet which has got clogged, or the
heater needs descaling.

Try the Mira website and search for descaling and/or the model of your
shower:


Thank you. I can't find a model number on the unit (it was installed
when I moved here, and I don't have any paperwork). It's an 8.5Kw
"Sport", which doesn't seem to be made anymore, and the current "Sport"
models have a different style.
--
Andy Mabbett
"The Internet is a reflection of our society[ ...]. If we do not like what we
see in that mirror the problem is not to fix the mirror, we have to fix
society." Vint Cerf