Appreciate the comments, guys, thanks a lot for your pointers! I've been trying to follow them and they've been quite helpful!

Here's an update. Today I decided to give the rear derailer a nice clean-up. I used a degreaser and a rag and started rubbing at the dirt. And damn, there was dirt. I managed to take out all sorts of grime and now I'm starting to think the previous owner wasn't the fine British gentleman drinking tea at 6 PM that I used to imagine, but more likely a dirty mother f...er who liked to blast through mountains of crap at 30 mph and also who never ever cleaned his goddamn bike. /rant off

So, to make the long story short, once I had done some cleaning, I noticed that the derailer spring started jumping more lively than it used to. Once I put back the rear wheel, I tried out changing the gears on the rear while being in the small chainring in front. There has been some progress. Now the derailer will go to the 6th cog without a problem (as opposed to the 5th before the cleaning), upon shifting to 7, it will hesitate a bit, but when shifting to 8, it will actually go down to the 7th cog. The story is basically the same with going from the 7th cog to the 8th cog. The only cog I'm (still) unable to select is the 9th. Going back to the bigger cogs is even easier.

What I've experienced today seems to indicate that indeed, cny-bikeman's theory about a foreign object obstructing the derailer from moving, must be correct, since I've made no other changes than cleaning up a bit. I've noticed there still is some 'material' left to clean up and I hope that I'll find a way to take out all of it. I'm also considering replacing the gear cable and housing to ensure more crisp shifting. I'll measure the chain length once more and if I confirm that it's indeed too long, I'll have it shortened.

About B-tension - I'd love to change that setting and see how it reacts. I even tried to. However, the bolt (currently in a very unscrewed position) seems to be rusted/stuck and I can't seem to make it budge. If you guys know of any nice tricks to release stuck/rusted bolts, please let me know, since I'll probably need the advice anyway when I get down to replacing the rusted bolts holding the cantilever brake arms to the brake bosses.

Anything else that I'm missing and you guys can recommend?