Re: 156 auxilliary belt ?
Hey, it's not that difficult. I've just changed mine.
You need a 5 mm Allen key and a 15 mm spanner. And a new belt.
Take off the top guard, it's on the LHS of the engine as you stand looking from the front, two Allen screws. Thread the new belt around the pulleys, as I've shown in the attached sketch.
You then need to push back the sprung idler pully, it's the small pulley in between and just in front of the generator and the power steering pump pulleys. Put the 15 mm spanner on it and turn it anti-clockwise as hard as you can to allow you to get the belt over the air-con pulley. I removed the top engine steady to make it easier to get a good grip on the sprung idler - you have to push quite hard. If you can't get enough grip, then you'll have to take off the bottom tray as RB said and do it from underneath. When you've got the belt threaded over all the pulleys let go of the sprung idler to put the tension on your belt. Replace the top guard (and the under tray if you removed it) and job done.