Sorted it in the end. No idea why it was doing it, but I put new discs on and it was still doing it. The new caliper (from scrapyard but looked fine) was exactly the same as the old one, yet it seemed to sit to close to the disc, causing it to grind on the edge.
In the end we jacked it up...
Make Mazda
Model 323f 1.8i Executive
Engine size/Type 1840
Year 1997
Mileage 146000
My nearside front brake caliper piston was knacked and causing the brakes to bind on the disc, which has been the case for a couple of weeks (had to wait until pay day to sort it). The pads were also knew...