Rendered and un-rendered both yield the same results. But yes, it did appear to be a problem with the chosen font – if I make it any other font the text is properly formed, even when editing. Thanks both for the ideas!
… and since this got no replies, I’ll update in case it’s of use to anyone. I was recommended Video Mill for the PC and Playback Pro for the Mac. I’ve installed Video Mill and am finding it very confusing to use, though it seems it’ll have the functionality I need.