Much appreciated everybody. I cleared the font cache, dumped the ID prefs, tried several other things, some of which came from other forums/websites, and for now it seems to have fixed it. I'm not sure which suggestion fixed it – in fact, it seemed fixed for a while before reverting to its unrult self, and then back again… so I will reserve judgement – but at the moment it appears to be working as it should.
Well, almost. As much as I disagree with Willi's comments – and I maintain that his way of working is clearly just HIS personal preference, not necessarily anybody else's – my problems persist when working with a text box that has more than one weight of font in it, or a variety of fonts, which have not been determined by character styles. I'm assuming this is a deliberate feature, and something which is new to ID. I only ever use Character/Para styles when setting up multi-page or text heavy documents, but it would appear that Adobe want you to use them as a default. I could be wrong.
Maybe Willi works on the ID dev team?
Thanks again all, for your suggestions and help. Still not sure if it's fixed