There's obvious radial movement (ie in the same plane that the disc rotates in), so I'm thinking its the bobbins that are worn. I fitted new discs around 3 years ago, I'm pretty sure I fitted new bobbins/washers, the lot, all sourced from James Sherlock - I'll be ordering some new bobbins this evening.I've done some Googling and read a few horror stories regarding the disc mounting pillars (the parts on the wheel that the disc sits on, and is bolted to) having worn away! Gulp.
There does seem to be a fix though, fitting washers between the disc and the wheel, although I'm pretty sure I fitted some when I did the discs, so hopefully its just the bobbins that are buggered.There's always something, isn't there!
Pete