Then the manuals wrong.
The tappings for the heat leak rad should be below the cylinder tappings.
Not at the same level as in your set up.
And the heat leak rad should be in a living space, not a cold loft where the rads output will multiply.
You may also get parisitic gravity circulation to the rad when the cylinder is heated via the immersion or the gas boiler.
(probably not yours as the immersion looks high up and no boiler seems to be connected)
I assume hi-lo stats are fitted and set up correctly!
Has the circulator thats sitting vertical been rectified?
The tappings for the heat leak rad should be below the cylinder tappings.
Not at the same level as in your set up.
And the heat leak rad should be in a living space, not a cold loft where the rads output will multiply.
You may also get parisitic gravity circulation to the rad when the cylinder is heated via the immersion or the gas boiler.
(probably not yours as the immersion looks high up and no boiler seems to be connected)
I assume hi-lo stats are fitted and set up correctly!
Has the circulator thats sitting vertical been rectified?