Thermostatic shower temperature problem

[R1ck]

Sirrus thermostatic bath mixer shower fitted brand new. Hot water gravity fed from cylinder, cold water mains fed (don't know pressure but seems v/high) Head of water from tank to shower head approx. 1.2m.
Problem
Switched to bath temperature OK - nice and hot. When switched to shower temperature falls noticably. Is it a pressure problem? What can be done to fix please?

 

[Softus]

Sirrus thermostatic bath mixer shower fitted brand new. Hot water gravity fed from cylinder, cold water mains fed (don't know pressure but seems v/high) Head of water from tank to shower head approx. 1.2m.

Oh dear.

When switched to shower temperature falls noticably. Is it a pressure problem?

Yes.

The restrictive hose and shower head result in the cold mains pressure defeating the hot supply pressure.

What can be done to fix please?

One of the following:

1. Provide balanced supplied to the mixer tap.
2. Fit a pressure reducing valve on the bath cold supply.
3. Fit a balancing valve on the bath hot and cold supplies.