Trevi Boost venturi shower problem

I have seen reports of inadequate pressure and cold temperatures - but my problem is that the water is too hot! I don't understand how this can be given the design of the shower valve.
Recently had a Trevi Boost venturi shower fitted. The plumbing appears to be correct - with feed from cold mains and top of hot cylinder. There is at least 1.5m head on hot tank. From new the water temperature was very hot - and turning the temperature control to "cold" still resulted in pretty hot water. The pressure seemed low and it cut out after a minute or so. I thought the high temperature thermostat was cutting it off. The hot water tank was at about 70 degrees. So I turned down the temperature of the hot water to under 50 degrees. The pressure is now better - so perhaps I was correct. However the temperature control of the shower still results in water which varies from hot to pretty hot - with no cool or cold water ever coming out. Additionally the hot water from taps/bath is now under desired temperature.
I don't understand why the temperature can't be adjusted to cold. I also believe my first hypothesis may have been wrong as if the temperature cut-off was the issue - any water which did come out should have been cold - and it was very hot!
So any explanations as to why a venturi shower should produce water which is too hot? And why I can't turn the temperature down to cold? And why the shower pressure/flow increased when I decreased the temperature in the hot water cylinder?
--
Matthew Barnard

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