Thanks so much all for the help :)
@plantladylin Yes, I see why the black dots could look like scale - but they were actually dark reddish/purplish goo that appears to have been secreted by whatever critter has been munching on the plant. Sadly I don't think that's the solution, but I'll check out the articles - thanks very much for looking them up! :)
@purpleinopp Good guess on the thrip, that's exactly the shape it was. But if it is a thrip it's not like one I've ever seen - it was a really bright red colour, and the damage also isn't what you'd expect from a thrip. It's a real mystery! As you suggested, I've removed the affected stem. So far I've not seen any other problem areas, but I need to do a more thorough check. Thanks for the suggestions!
