red and black wires can go anywhere right ?

Next time, use Molex

The trouble is, how idiot proof must I go ? Should I then have a different connector on the mains supply to the psu, etc etc.

I want to avoid connectors as the equipment is going to the tropics, and I like to spray everything with clear laquer to waterproof it.

These guys that swapped the red and black over , wire up light boxes and high voltage neon lights, its a big signage company that connects up LED strips etc !!!