330 aux belt - tool

Hiya I want to change the 2 engine auxilliary belts. It looks like i need to remove/loosen the tensioners but I dont seem to have the right tool to fit them.
I have a T40 bit that looks right but too small. Thats the largest of these i have. I dont mind buying another tool but would like to know for definate what it is. Can anyone help here please. Thanks, Andy
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