Well thinking long and hard and looking at what the manufacturer had chosen as suitable, I found that two strands/cables were judged to be enough for 12V 20A. A quick check with the 24VOLT.eu site told me that it was quite OK. So I could use that as template for the rest of the setup. The site turned out to be quite useful and the link is here:
http://24volt.eu/kalkylator_kabelarea.php. Click the UK flag to change to English.
I also decided to include one single 3.3 V connection on the outside to be prepared for any circumstances appearing. Had therefore to change the label so the rear of the PSU now looks as follows:

But it is not yet possible to put the lid back on - of course the load resistor for the +5V line was not in my store so have to buy it next week ...
While waiting for that to turn up, it is high time to try to understand the electronics setup and with the following walkthrough of the bits I hope to get some feedback if I am off track so to say.
The base is an Arduino Mega 2560:

It is fed using the USB connector to the left at loading and setting up time.
When done, the USB connection can be removed as the control board called RAMPS is to be fitted as a piggy-back board on top of the Arduino board. This contains the controls for stepper drivers, end stops, extruder heater, hotbed, etc.

The USB cable is not required as we connect the two +12V lines to the lower left for the two circuits.

Thereby the Arduino board below seems to be fed from the RAMPS board. at least it looked like that when inspecting the connections.
Then add the SMART-adapter for he display board on top of the RAMPS board to the right.

Now we can attach the display board with it function selector in the SMART-adaptor connectors (quite short cables between those - hmmmm).

And finally, five small stepper driver boards is fitted on top of the RAMPS board. I placed one of the five in the upper row to the left.

This is supposed to be a job to be done with caution as if the stepper boards are turned the wrong way there is a risk for the whole setup to blow, all the way from the Arduino and upwards!
So, this is what I can see from just looking at the parts, is this correct? Is there a way to ensure that the stepper drivers are fitted correctly, I mean something particular to note at assembly time?
Now back to think about the best place for the control display .....
/Peter