Power Shower - only hot water

[baggieboy40]

Hi, first time poster here.
I have a shower and only hot water coming out... Hot water is through a pump, cold through the mains. Water pressure is fine when the setting is towards hot but as you turn it down to reduce below 1 million degrees it goes to a trickle and remains at lava temp levels. My basic and unskilled assumption is that there is no cold water getting through... anyone have an idea how this would be fixed???

cheers

 

[seco services]

can be a non return valve stuck, filter blocked.
has the cold mains pipe to the shower got a pressure reducing valve fitted ?