Changing the belt, as stated by Brian above, doesn't need the alternator to come off.
If you DO need the alternator off, and it's a TDCi engine (90PS turboD) then try this:
1. Remove two plastic lower covers from below engine on r/h side (Torx20 and 8mm socket).
2. Spanner on the tensioner centre bolt (up towards top of engine at rear), pull down hard to release belt.
3. Undo and remove three 8mm bolts that link alternator to it's drive assembly, then remove three 10mm bolts holding drive assembly to engine and remove the drive. I thought I could get away without doing this, but I couldn't :-D
4. Remove air intake for intercooler (two Torx15 screws). Undo air hoses from intercooler, remove four 8mm bolts and lift intercooler off.
5. Disconnect wiring from alternator (one plug, one 10mm nut on main battery lead). Some folk would tell you to disconnect the battery instead but as you need the lead off the alternator, just remove it and tape the end up while you work. Also means you won't lose the radio code ;o)
6. Remove top and bottom 10mm bolts securing alternator to its bracket and drop it out from below the engine. You CAN get it out from the top, but it's a struggle. Note that the engine bracket is slotted so once you get the top bolt part-way out that'll do, it doesn't need to come right out.
I think you can swap the alternator without taking the intercooler off, but if you're lying on your back in a puddle like I was, it may be less hassle to do it this way :o)
Wedg1e, January 2008