Thanks for the responses everyone, been quite busy lately!
They are decent discs with hardly any wear. Think I'd rather get them skimmed if that's the issue. (I can actually see pad deposits on the discs!)
The rubbing doesn't happen on the road, it's only at the track. I think I noticed once when coming off a roundabout quick, but otherwise you can't hear it. It's definitely not anything between the disc and shield, it sounds and feels like the tyre rubbing on something when going round some corners at the track. The lines on the tyres are exactly the same on both sides so I don't think it's something from the road as they are too uniform.
Any other thoughts?
Cheers,
Adrian