I've asked around some 4x4 off-road friends and they have said that it is a common problem with the diesel Shoguns (TD) that it cause be a worn pulley. They hadn't heard of it on a Petrol v6 though. It can be fixed by replacing the pulley and using thread-lock on the bolt.
I've managed to torque the bolt up to near it's prescribed settings and I will see how it goes over the next few days. A replacement OEM pulley cost 75.00+ GBP which I would like to avoid if I can, but I would also like to avoid blowing the engine. (IMG:style_emoticons/default/sad.gif