Hi Mike
Grumpy_Mike:
and that their effect on the system voltage should be almost instant after power is applied ?
Not too sure what that means.
that I have a faulty power unit
Yes that would be my guess as well. Just remove all the load to check but it sounds lie a faulty supply.
Of all the parts in a system the power supply is nearly always the most unreliable.
I am assuming that any effect that any components of the system could possibly have, should be reasonably constant and should exist from shortly after power is applied, and not degrade the power like this over a period of time.
I have removed ( switched off with toggle switch ) the load and the power supply is now reading 11.5V