The flies are there as you are keeping the soil too wet. Allow it to dry out between watering and keep it damp, not soaking - you can tell it is too wet as the bottom of the Draconia is wet and the soil is black. I would let the soil dry off completely, till it is really dry then take the plant outside and shake off all the soil. Get new compost and water the plant. The should hopefully get rid of the flies and keep the plant well. They really do not need much watering over winter.

This webpage info may be useful to you:

http://apps.rhs.org.uk/plantselector/plant?plantid=2847

Your plant is Dracaena marginata 'tricolor'. If you think its got too tall, you can start a new one, but best done in spring, when you chop off the tops (about 6-8 inches), strip off the lower leaves till half the stem is bare, and pop them in a bottle of water on a windowsill. Keep the water topped up and after about 6 weeks, they (or it) will have rooted, at which point you can pot them up. The parent plant can be cut down a bit more - wherever you cut, two or more new branches will arise.
Follow Kildermorie's advice re watering, etc.