THANKS VERY MUCH for these awesome replies. I feel much more confident to attack this job now.

I must especially thank BillyBob for his excellent reply quoted below

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The easiest way is the proper tool of course! But I use an old serpentine belt around the fan pulley, clamp it it place with vice grips and it forms a very effective strap wrench to counterhold the pulley in place! The 8 MM Allen head bolt is fairly easy to get out if you take a 1"X1/4" flat steel bar and weld a 1/2" piece of Allen wrench to it. The best way is to drill theexact size hole through the steel bar and then weld it from the backside leaveing the proper 1/2" protruding. The other method is to cut a standard allen wrenche's short leg so that only a little over 1/2"or 3/4" of the leg is left then you can get a piece of pipe over the long leg and turn it, I use the hollow steel handle from my hydraulic floor jack its heavy walled and about 18" long so it works perfect. The real trick is to get the Allen bolt's head very clean using carb or brake cleaner first, then get the allen wrench seated all the wat into the hole, the bolt has blue locktite coating on it so it takes some force to break it free. Once you get the Allen wrench seated correctly you can slack the serpentine strap wrench to rotate the fan pulley hub, its easier to do that that to reposition and reinsert the Allen wrench into the bolt head. Good luck! If you don't already know the tool kit lug wrench handle fits into the black plastic cam block hole at the top of the tensioner arraingment. You insert it and lever it a little to take the tension off the securing bolt that passes through the head. Take off the 13 MM nut, lever the cam block enough to lesson the force on the bolt and then push the bolt rearward enough to get it's end past backwards out of the plastic cam block, them you can completely release the tensioners force. If you simple replaceing the belt it can be done without removing the fan, you can just work it through the fan blades with the fan duct loosened.
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thaks mate!