Just received my new belt tensioner (Pulley) and on checking the play in the old one compared to the new I found a hugh difference. I am now convinced that this is causing the belt to shred at high RPM i.e a small amount of play at low RPM is greatly increased at high RPM.
A warning to all you guys out there check this ASAP to avoid the misery of having to remove everything to repair the crankshaftsensor wires after the belts break.