A month of wasted emails to the manufacturer have brought no answers, and I'm rapidly running out of stores to visit for assistance.
The model of external hard disk is Medion 250GB USB 2.0, COMPATIBLE TO USB 1.1 HDDrive2GO MD 90046.04.
Info on the reverse of the unit states the requirements as: Input: 12V === 2A --- +-
The only piece of useful information gained from the manufacturers is that we require a standard round pin connector AC adapater.
I even purchased a multi 1000mA Mains adaptor today, after being advised that this was a 'catch all' - but
that didn't work either...!