Well you've described the causes Rich - high ISO and low light are the main causes of noise in your images. Noise will also be seen most in the darker part of your images.
I have to say I often use ISO 400 and usually don't find the noise that noticeable, in fact it can often resemble grain in film with modern digital cameras which isn't always unacceptable or unsightly.
Do you try running these shots through noise reduction software?
