I took apart the power supply by sawing through the shell on the three clear edges carefully
so I could then crack it open.
After lifting out the board I removed the two screws holding the internal connector and on turning it upside down noticed that the centre pin was not isolated due to a surface mount resistor in there connecting to one of the other terminals. Follwing the idea of isolating the centre terminal, I desoldered the resistor, and bingo! the screen works perfectly now, and it is a proper fix.