Well - there you go! I took off the needle clamp thingy (there was a thread guide in there too, bit fiddly to manoeuvre back on), cleaned it thoroughly and when I put it back together again it was fine! I don't quite know what made the difference as I didn't get out anything "big" from in there, it was all just dark and generally grimy but for whatever reason that seems to have fixed it. Perhaps that little thread guide was throwing something off just a tiny bit at the top of the needle and the re-jiggling it all to get back into place set it straight again!
Thank you so much for the tip - I'll keep it under my hat for the future, for sure - any trouble with the needle and it's not the usual culprits then clean out that needle clamp. :-D