14c is not at all too cold for a fukien in winter. I keep mine at night temperature 7c. On sunny days it can go up to 12c. I think moving the tree to the extension would be a lot better for the tree than the current position. It will also significant decrease white flies reproduction speed.

Several years ago I had a major problem with white flies and could not get rid of them. Somebody on the forum here suggested getting a product whose name I can't remember. It is yellow cards with very sticky sides, hung from a branch, which attracts white flies.

I hung one up and in a couple of days it was covered. I hung another up and it caught a lot more over the rest of the winter. Next year there were some. Last year and this year, none.

I just found a similar thing in Amazon by searching "yellow sticky insect trap". It involves no poison or harsh chemicals on the trees.

Did I miss to mention the sticky traps!! They are all over my greenhouse. Excellent for white flies and very good to keep track of any flying pests.

Can I ask a quick question.

In the photos posted at the start you can see there are a few roots (or look like roots) that are poking out of the top soil.

Should these be covered, or is it fine for these to be like this?

Tjat is totally fine. In bonsai we want exposed roots to make them look old and well established.
Google the term "Nebari Bonsai"

Not that your roots makes a great nebari, but still no issue for the health of the tree. If you make the tree survive a few years, you can start to work on a better looking nebari.

Hope it survives.

It is a nice looking tree and has potential.

Not many leaves dropped to far, those that have had black spots on them anyway. The black spot situ doesnt seem to be getting worse.

Seem to have got the white fly under control, and when compared to Boxing day there is a lot of new growth.

I am not sure if new growth in the middle of the winter is a good sign. My tree has not grown since mid october and I would like to keep it that way until the sun is back in March or April.

Growing when starved for light may empty the trees recources and make a very weak tree come spring

But as said before, I don't have enough species specific experience to be sure. Let us compare our trees come summer. There may be something to learn from it.

14C is not cold. There is no need for extra light, keep away from heat lamps.