I have a 4-5 year old clump of Chusquea culeou (Chilean feather
bamboo). Its about 3-4 feet in diameter and now the highest shoot is
about 9'. However, all shoots are at about 40 degrees from vertical,
including all the young new shoots. It does not seem to want to grow
upwards. Hence it is unsightly and taking up a lot of one bed. The
centre shoots are also looking unhealthy.

As far as I can tell it is being fed and watered well, doesn't get too
much heat, so I cannot figure out what is wrong. Any ideas
appreciated.

It does actually tend to have that sort of habit in the wild when it becomes congested. Have you tried thinning it out?