OK, I've been playing in the garage and managed to suss this one! It's really easy.
For those with the same problem all you need to do is remove the main stand spring ( with the bike on the sidestand - watch your fingers!) take off the curved metal plate the top of the spring hooks through ( the plate just slips on a hook on the main stand). Drill a 6mm hole about 6-10mm above the existing hole and refit the plate and spring. No more sloppy stand!! ( if it's bad it's probably worth fitting a new spring at the same time).