Posted: 20/01/2013 at 22:43
You could give it some support by way of a cane and it looks like it may fare better in the next size up pot although now isn't the best time to re-pot, wait till spring. I'd also advise moving it away from the radiator into an area of the room which gets more direct light, so long as it isn't in a draft ie by the door it should be fine.
I had one which grew to about 6ft, shame you've lost the bottom leaves but an added bonus...your plant has started to produce side shoots.
It's difficult to say how often to water cactus plants, another poster may give more helpful advise...mine get a water 2 or 3 times a year but it's a good soak and they are stood in water for the day. It's usually when I can remember, they get a good feed at the same time.