The message
from David W EEE Roberts contains these words:

Despite the grotty cold weather my last couple of tomato plants seem to be
surviving in the cold greenhouse and they are still slowly ripening
tomatoes - presumably when the greenhouse warms up on sunny days.

I also have a few green tomatoes indoors in the warm in a plastic tub along
with ripe tomatoes and a ripe banana.

Strangely, the ones in the greenhouse seem to be doing better.

One of the plants is actually putting out side shoots, although neither
plant is flowering.

Has anyone ever overwintered tomato plants (or cuttings)?

Yes, but IMO it's a waste of time as there's not enough of the right
kind of light. Tomato ripening is, as I understand it, triggered by
shortening days.

It is the end of November now, and I see to recall that you start early
tomatoes in a heated greenhouse around February.

Correct. Or earlier, indoors, for an unheated one.

Perhaps I should bring in the side shoots and grow them as pot plants?

Well, something to try - remove all shoots and skewer a hole in one eye
of a potato and poke a freshly-cut side-shoot into it, and plant it.

I did this a lot of years ago (20?) and had a useful but not spectacular
crop of spuds and a normal crop of tomatoes.

Something not to try... tomatoes grafted on belladonna root are
poisonous! (I have friends at the John Innes Centre who specialise in
Solanum and allied genera.)