unexpected power supply behavior
Okay, given the comment that it's just a normal UK outlet, I'm going to take a wild-a** guess and suggest that your outlet may be wired incorrectly so that the wall outlet switch is breaking the neutral and thus the Y capacitor in the supply is continuing to conduct some current to through the ground connection and hot. 

This is based on some assumptions about the UK residential electrics that may not be valid, and I'm sure a right-pondian correction will be supplied if necessary. 







simulate this circuit &ndash; Schematic created using CircuitLab