T4 2.5 ACU petrol timing belt replacement - tensioner adjustment

[ericcs]

I,m in the process of changing my timing belt, water pump and tensioner. I finally managed to stretch the belt over the tensioner, but am unsure on how to tension the belt, there is a big black nut under the bolt, and a tab which is spring loaded. when you push this tab it tightens against the belt, but just springs back to it's normal spot.
any advice greatly appreciated. there are youtube clips, but their T4's have a different tensioner, plus an extra idler bearing.
cheers
eric