Update:

After thoroughly lubing up all components in the rear the creak is still there. The e-brake is a possibility as the noise goes a little when the car's parked up and I mess around with the e-brake, and move the car. The actual sound itself definitely sounds like metal on metal creaking though (as would be expected from torn stabiliser bar clamps, or an issue with springs) or a moving bushing.

The bolts on the rear shoes are incredibly corroded and the socket set I have would likely give up before the bolts do so I'll leave it to my mechanic to look into. The rear pads could do with changing in any event so I'll leave it in for that and ask him to clean and re-seat all e-brake components while he's at it.