Where is my belt tensioner pulley?

I read the RAVE manual but I cant find the thing mine looks different, which one is it ? https://dl.dropboxusercontent.com/u/82920844/Pulley Belt.jpg

I should add that I thought I knew what it was but the bolt in the middle its just that, a bolt that I can tighten or undo. I refer to the 15mm bolt, The smaller thing it attaches to has a 13mm bolt, I can also undo that or tighten it up. I thought there should be something that is spring loaded.

OK cancel this, thanks. I just needed to be brave and tighten the 15mm nut and keep going until it swivelled & released the tension. Doh!

:hysterically_laughi enjoying yourself?