Do what the old time Eskimos in the Arctic did and that is to generate heat from the food you eat and then keep the heat close to your body by having good insulation. There is a very good reason why seal blubber is still eaten by native people all over the North. (Good luck trying some if you can find it)
I've seen old timers walk around in -50 degree C weather with a good anorak and bare hands because their body core was warm. In real cold weather you need to eat a lot of fat and then trap the heat with layers (thermals, fleece wool or down and then have a windproof layer on top) Wind increases the rate of evaporation and lowers the "apparent" temperature that the skin feels. Hats and gloves are essential. Drink lots of water and watch how much coffee/alcohol you drink as it will reduce the circulation in your extremities. Stay dry. Sweating makes you colder. Change damp socks/boot liners and gloves regularly. If you wear boots with liners you need to take them out of the boots to dry them properly. A spare pair is very useful.
So, to sum up, eat high energy food and keep the heat in. There are only two ways the body generates heat and that is from the food you consume and exercising. If you can't/don't move then the internal furnace has to do all the work. It doesn't matter if you are in London, England or Shaktoolik, Alaska the principle is still the same. Stay warm.
Thanks to all contributors. A few hours on PPRuNe has been much more helpful than bl**dy Google, so thanks to all.. You've given me lots more ideas to explore, for which I'm grateful. On the extremities, I think I've sorted that with thermal gloves, but I will try the idea of wearing a very thin pair under the main gloves. Thermal socks and decent, (but not branded rip-off) boots are currently OK. The head gear, however, is worth developing. Current head gear is warm, but not windproof,so I need to look for wind/water proof headgear. On the coats/other apparel for the body (looking at Juud's idea - can't call it a coat!) , I will explore your various leads. Thanks again. Clearly I was right to get rid of my lined Barbour, and the sheepskin coat, and I felt virtuous that I gave it to the charity shop.
On offer for 34 quid, two villages down from here.
I´ll be happy to nip out tomorrow and get you one. PM me your size and address if you want. And don´t be all horrified and British about a complete stranger doing something for you. We´ve shared PPRuNe for long enough, it´s no problem.
Postage is expensive from here though, so you gorra factor that in.
I'm amazed nobody's posted the following yet, I assure you they're NOT "urban myths"
1- 30% of heat loss is through your head....those fur-lined hats with the side-flaps that fasten over the top and the turnup round the back, - we used to call them "lumberjack's hats ".....in foul weather, pull down the tur-up and fasten the flaps under chin.....that's ears and neck sorted.
2 - War-surplus stores used to sell "Ex-RAF silk inner-gloves" Equivalent probably still available from a good climbing or sailing shop.
MUCH better than the nylon ones which don't wick away the sweat like thereal thing....apparently they were a godsend to Aircrew in unheated planes.
As regards wind/waterproof outer clothing......sailing gear! paradoxically, now's probably the time to buy as the racing fleet and most cruisers are laid -up for the winter.....Chandlers will get in the latest trendy gear next spring.
If you are near a store that sells military surplus, see if they have any of the wool NATO khaki commando style sweaters. The one I have is long enough to go past my butt and I've been through some winters with temps down to -25C with the usual layering and good quality jackets.
Also agree with cockney steve 100%, re the " lumberjacks hats ". I've seen them in surplus shops. Add a wool or thermal material scarf too.
Thanks to all for your help and ideas, and for the offer to go shopping for me. In the UK, the sales have started, and despite my earlier intention not to buy heavily branded stuff, I have bought a heavily reduced heavily branded down filled coat - the longest one I could find. (I won't give the buggahs the publicity)
One of the problems with some of the suggestions has been my inability to try things on. Whilst I liked the ideas on here, some of the stores required me to travel 200 miles, in the UK (or to USA/Norway/Russia etc) to try things on. Not possible, I'm afraid, and I do not like buying things without trying them on. ............For instance, I'm not really X Large, but in quite a few fittings, I've found that X Large is required to be able to get the layers on.