Hi Sicco, thanks for the reply. At the moment, I don't think I'm in a position to go beyond the standard library, unless there is a very straightforward way to implement this. I don't think I have the time and/or skill to do it. With this...
Thanks for the suggestion. I had another look, and it doesn't appear that there is much noise coming from 50 Hz - see this FFT:
here's some data:
so it looks like the noise is coming from a range of frequencies.