Low pressure/temerature problem on Triton Opal shower ?

Hi

We have a Triton Opal shower fitted in our rented flat. We usually get a good temperature and pressure at around setting 4-6 and have never had to turn it up past this point.

In the last week the shower has become cold/lukewarm at setting 4-6 and needs turning up to 8-10 to achieve an acceptable temperature. At this level the pressure is so low that we cannot have a proper shower (slightly more than a dribble).

Our landlord has been out and refuses to send an electrician as it is still "working".

We would be really grateful if someone could help diagnose the problem before I spend money getting an electrician out.

Many thanks
Andy