Been out for a drive aiming to track down the noise, and now I think that it's more likely to be the front brakes. Looking at the discs there's a slight lip on both the outer faces and I don't think that the whole disc is being equally swept by the pads. My current theory is that the discs might not be 100% true, which is why i hear it when lightly braking, and not when braking more heavily. Does this sound sensible?