I resorted to SwitchResX, which managed to successfully set the 3007WFP back to 2560x1600.
Thanks for the tip, I didn't know about SwitchResX - just downloaded it and my 3007WFP working perfectly now.
I've been working with the very helpful CJ at Apple support, and he's on the case trying to resolve the 30" monitor issue. In the meantime, SwitchResX has brought back my 30" window into data space.