I've lost the power steering pump etc. Thinking like any other car it'd simply be a matter of swinging the alternator back a bit to tighten it, I thought it'd be an easy job. Having had a quick look though, it appears to be automatically tensioned by a big spring?
I assume you checked the tension of the belt. Is it possibly old enough to be stretching or smeared in some gunk, in which case the extra tension probably wont help.