Awesome thanks a lot, I'd not seen that! It looks very in-depth (which is what I wanted :), I'll have to have a good look through it.

Edit: Wow, I'm just skimming it at the moment, I'd never heard of 'Magnetic Particle Filtering' before, that is really amazing!

Also semi-related, I just found this https://en.wikipedia.org/wiki/Magnetic_anomaly_detector. Apparently they degauss submarines precisely to avoid that :)