Well if it's just the straight forward alternator belt it can be done in a few seconds.
**Disclaimer**
I am not responsible for any damage caused to person or vehicle due to following the directions given.
Cut your old belt off, Hook the new belt over the alternator, then as far over the other pulley as you can. Then give a quick flick of the iginition. Job done.
May I just state this was taught to me by an ex Vauxhall mechanic who was then my tutor when I was at college doing mechanics.
I have used this method many a time but only on the older 8v. It isn't possible on the 16v or with powersteering.