Slow running Mira advance electrical shower

Please can anyone think what might be happening here.
The shower used to give a powerful boost of hot water almost immediately on starting. Now what happens is that the water starts on full, then there is a clicking noise and it changes to a trickle. It sometimes goes off and the low water pressure sign comes on, or it runs at low. If you press the High button again it comes back on. However it is never as good as it used to be. I have checked the hose and head for blockages but they seem to be clear (we have soft water anyway). The mains water seems fine. Any ideas?
As far as I can remember it started around the time we had a plumber in doing the kitchen but I can't think of anything he could have done - eg the cold water mains tap seems to be full on.
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