Aux drive belt tensioner

I'm having problems getting the drive belt back on. As i push the tensioner wheel back, by turning the spanner, the nut is turning, so i can't get the tensioner to move back far enough to get the drive belt on to the bottom pulley......am i in deep trouble or is there another way?
rich

Update - i used a crow bar between the chassis rail and the nut, at the same time as ther spanner. This used up both hands so had to enlist my mates help. As i held the tensioner nut with the sopanner and crowbar, i was able to move it enough for my mate to put the belt on te pulley. So all is OK!!!!!

I made a lever by tapping an M8 cap head screw into an old tyre lever. The head of the bolt fits nicely into the recess in the centre of the tensioner pulley and the bar can be pivoted on the fixed idler. (This means the lever has to be pushed towards the front of the vehicle, not pulled rearwards). This gave me about a 4:1 lever ratio which made moving the tensioner fairly easy and the beauty is that the lever cannot jump off the nut as it can using a spanner. Did it single handed with no grunting.

I did describe this earlier, search TD4 drive belt, (3 Mar 2009).