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Thanks for the suggestions; I tried using the Suppress Frequency Bands, starting with the peaks at 50, 150 and 200Hz, but it made little difference. In the end, I tried a trick mentioned on the Audacity wiki, of decreasing the volume of the noise sample before sampling and then suppressing - that wa...

You could look at the spectrum of the noise and see if it has line frequency components. If so, you can always use notch filters to get rid of it. (for US, usually multiples of 120hz) I have a similar issue, in that a voice recording that I made has a very noticeable hum, caused by what looks like ...