Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On

Observation:

Check the Water Flow Limiter (located at the base of the Diverter Valve).
It is correctly adjusted?
Note 1:
Turning the adjustment screw clockwise reduces the Flow Rate. Turning it anti-clockwise will increase the Flow Rate.

Note 2:

The Water Flow Limiter Adjustment is only normally adjusted to reduce the Flow Rate to give a greater temperature rise at the users request.