Triton Rapide Shower and Low Pressure Light Shows?

[Brian]

A 'just moved house and it doesn't work properly' question.

Shower unit is a Triton Rapide 4 plus. It worked fine for a few days but now runs cold with low pressure light lit.
I have run it with max temp and flow slows (shower head removed) as it should and then increases as the temp control is turned down. Shower head is clean.
When cold flow button is pressed there is a good strong flow, but cold, I don't do cold showers !!!
When next heat button, Economy, is pressed then flow is just the same, good and strong but the low pressure light comes on and cold water again. Still cold even when temperature dial is turned up, flow slows of course.
Exactly the same for the High setting.

Tried different times of day but no change.
So, what might be the problem and what can I do about it ?
Many thanks.
Brian.