I complained when we bought the Commodore that the steering got terribly hard when you were asking the power steering to work harder than idle, and I finally got around to investigating it this weekend. The markings on the tensioner weren't actually pointing towards a stretched belt, but in fact the other direction - something several forum posters reckon indicates a weak tensioner.
Deciding to bet some money on them being right, I went out and picked up a brand new tensioner (SCA Part #38195) and another belt just because, and whacked them both on in the space of about 30 minutes. Took it for a test drive, and it's not perfect but is significantly better. I know that the Mercedes is extremely intolerant of turning the steering any distance while the pump isn't spinning, and it takes it a good few lock-to-locks to get it to shut up and work 100% again, so maybe it just needs a bit of time?
Here's hoping it's not going to need a pump or something, as it's definitely not short of fluid as I've checked that multiple times. It's almost due for a service again, so I'll have to set aside some time on a weekend to do that too - or if I'm too busy I might just pay someone to do it.