it's dead easy, just unplug the battery first, and make sure your connector is the same before you go to all the trouble of doing it
One problem I had was that the hole in the alternator that the tensioner bolt goes through was smaller on my new alternator. I need to drill a better hole in the alternator rather than using the dodgy nut and bolt that I stripped with a tap and die!.. they were both A127 alternators. I think you can get different pulleys on different A127s as well, so make sure you get the right one. Those are about all of the pitfalls that there could possibly be, and it took me 30 minutes. Think I could do it in 10 now.
Thanks for the tips chaps, ive also been playing around the dipstick and need to check that I havnt just knocked a connector off before I start going to all the trouble!!!