Just done this job tonight folks and wanted to post and update to this thread as I feel poor old Freeme wasnt getting where he wanted and the thread died a death.
I had exactly the same problem as him. Initially when I turned the tensioner pulley anti-clockwise, us7ng my trusty 24mm spanner, all appeared well, the belt went slackish but I noticed not slack enough to get the belt off. Bit more pressure on the tensioner....still not slack enough...and by now ive realised that the centre bolt is in fact turning and I eventually reached a point where I couldnt get any slack in the belt at all.
I had some ideas at this point, mainly from LR guys info on removing idler pulley located below however I wasnt happy with doing this with the belt under tension as this may have put excessive load on the idler pulley bolt as it reached the last few threads, and of course I didnt want to f*$k it! Cut the belt?? Chickened out that one too as that would be a show stopper should I fail to get the tensioner functioning to fit the new belt. Now as LR GUY state, the bolt head on the inside of the pulley is a 15mm head. Now with a lot of patience and ****ing about I was finally able to get the open end of a 15mm combination spanner on this head with the belts fitted as normal. The only way I was with the spanner basically lying horizontal with the ring end of the spanner pointing towards the back of the car. With a tommy bar through the ring I was then able to tighten the tensioner centre bolt.......MOST IMPORTANT POINT, THIS IS A LEFT HAND THREAD.....so you are still pulling the 24mm spanner anti clockwise whole bracing the other end as above. With this done, much to my relief, the tensioner then slackened normally by turning the 24mm head anti clockwise. I hope this helps anyone who has the same problem.
Note that everyone seems to give differerent opinions on how diffcult this job is. To be honest, once I got the tensioner functioning, it took about about another 15 minutes to fit both the air con and auxiliary belts so I would say the job is pretty straight forward. I didnt remove the wheel and the only cover I removed was the inner wheel arch, this is more than adequate for access to the belts. I would say this is definitely one of those jobs that requires a steady hand and a bit patience. If you get ****ed off then leave it and go have a cuppa then try again.
Finally, please see attached picks which show tensioner pulley and ive tried my best to show both hex's. Also for a bit of a laugh ive attached a picture of my AC belt so you can all see what lazy fckr ive been up til now by not changing it.