Venturi shower pressure problem

I posted a couple of weeks ago about a problem with my venturi shower, in that the water wasnt hot coming out of the shower. It seemed to right itself but now its gone back to being cold again.
Ive got a feeling it has something to do with fluctuations in the mains cold water pressure. When the shower head is at its highest it comes out very cold, but when the head is placed lower, below the level of the venturi valve or in the bath, the water comes out nice and hot.
How likely is it that my mains water pressure has changed?
Coincidentally, my kitchen cold water flow is slower of late, so maybe it is a problem with my mains, even though my bathroom cold taps dont seem to have changed (and they are direct off the mains also). My only other inclination is a blockage of some sort.
Is there any way i can rectify this?

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