Thanks for all the recommendations. The recommended plates aren't too expensive but not cheap either. Are they something that lasts nearly indefinitely, or do the diamond flatting plates need to be replaced from time to time?
And could some coarse silicone carbide paper (wet & dry) be used if stuck down to a flat marble or glass slab?
I have some older stones that are quite badly dished (cheapish dual sided #1000/6000 and a 400ish/??) the higest grit side is still reasonable shape but the coarse is not so good. But more recently got some King S (#6000) and G(#8000) grit stones that are still pretty flat, but don't want them to get out of shape.
So my intention is ideally to flatten the dished stones and maintain the newer stones that are still in reasonable shape.