MIXED FLOW SOLUTION
If you have a Gravity Hot Feed and a Mains Fed Cold Feed then you have a Mixed system.
If your mains fed cold is higher than the gravity hot side and you are having issues due to this imbalance you will need a Shower Power Booster to give the gravity hot side a boost. If the mains fed is significantly higher you can add a pressure reducing valve, or a better solution would be to add a second pump to the gravity hot side, giving it a double boost.
Diagram 11.1 Automatic & Manual Pumps (SP21S) linked together and installed on Gravity Hot Feed giving a Double Boost to balance against a mains fed cold supply.