For anybody else needing to know, I pulled my finger out and did a bit of investigating. Looking down at the pulley, there is a bolt with a 12mm head protruding from the face of the pulley a little, and a nut that sort of sits flush in the pulley that sits on a stud where the adjustment circle is (on the pulley). In the centre there is a hex head hole. What you need to do is loosen both the bolt and the nut, fit a 10mm allen key to the centre of the pulley tension up the belt by levering the allen key anti clockwise and tighten up the nut and the bolt again. Due to the close proximity of the pulley and the fan/radiator shroud, it's a a pain in the bum, but it's not difficult.
