Alternator belt tensioner, lug keeps snapping

[MarkA67]

I'm trying to change the alternator belt on my car, a Scenic MK2 1.5 diesel. I managed to get the belt off fine, but I can't for the life of me get the new one on as the lug you have to turn to unload the tensioner keeps snapping! I've watched videos on YouTube and it appears you have to turn the tensioner until it's almost touching the crankshaft in order to get the belt on.

My tensioner is getting about 3cm away from the crankshaft, pretty much at the 6 o'clock position but the belt is still nowhere near going on (still a good CM away.) Putting anymore pressure on and trying to take the tensioner further causes the lug to snap clean off.

I've tried putting it around all the pulleys first and then the tensioner. I've also tried leaving the crankshaft till last, and the A/C compressor till last, but it's not having it. I took a 1199 6PK belt off, I'm trying to put a 1205 6PK belt on. You'd have thought that extra 6mm would've made it slightly easier, but apparently not!

Is there a specific art to doing this? Any specific tool? (I'm using a socket, extension bar and a 1/2 ratchet) any other way to untension it besides the lug?

Thanks in advance, god I hate cars!!

[mickthemaverick]

In this video he seems to turn the tensioner beyond the 6 o'clock position: