Right, I'm pleased to have started some discussion anyway and get some ideas - thanks a lot for the replies so far! I'm definitely looking to keep out on the road bike right through the winter this year and to that end I have a) sold my turbo trainer and the bike that went on it, and b) bought (second hand) another road bike for the winter with hydraulic discs and clearance for wider tyres. Last weekend I even decided to take it out for a few hours knowing that it would rain heavily from start to finish - there were some absolutely biblical downpours and flash flooding. Oddly I loved it all.
So back to the cold... I'm not instinctively good at wrapping up warm, so my plans for kitting myself out are to make myself start sensible from the start of the autumnal chill (so now), and see how far into the winter those measures get me before I need to add to them. My starting point is going to be:
1) Keep the head warmer: a thin helmet liner/hat (don't have much on top nowadays, and what's left is always shaved short!). I'll add a neck warmer/face cover a bit later in the year too, which reminds me to order one...
2) Keep the core warmer: more layers than I normally go for, with a windproof jacket/gilet on top
3) HotHands warmers slipped inside the gloves (my fingers are always the quickest thing to go in the cold and will only come back with heat applied so I'm looking forward to trying these). I did try some of the heated gloves that Amazon sell for about £70, though I was a bit underwhelmed for the price and didn't like the fact that they had bright lights on. Also too bulky really for cycling so they went back.
4) Overshoes every ride from now on, and then adding the foot version of the HotHands above when it starts to get colder.