that's not done yet, but the shower seems to work as before.
the mixer replacement costs £100, so I'm not keen unless there's
good reason. anyone know about these things?

Assuming the Trevi Boost is a pumped power shower, I believe you are
actually committing an offence by using it connected to the mains supply.
Under certain, admittedly very unlikely, circumstances, you could
contaminate the public water supply by reducing the pressure in the water
main below that of the surrounding ground water.

You must not pump your mains water supply without certain precautions, which
almost certainly have not been taken.

Christian.