Dropped in to see my mum yesterday and she said the shower was playing up.
The symptoms are with dial #1 on hot (one step down from high) and dial #2
on 7 (of 9), the pump runs and the water gets hot. However, the temperature
varies noticeably and after running for say five minutes, the water goes
cold and the pressure drops. My mum has also said that it has stopped
completely a few times.

Diagnosis?

I assume a shower like this consists of a heater, a heat exchanger, a flow
control valve, a thermostat and a pump.

Cheers, Rob.