Rob..question for you. It's an old propery..isn't the fact that things aren't level part of its' attraction? But I know what you mean!
Most of the cracking will have come from movement over the years, shrinkage in the ceiling joists etc. Old properties usually had them made from elm or oak. More often than not, elm. How much upheaval can you stand? If you really want a flat ceiling then take down the old one, remove the old joists and replace with new. Job done. Unless it's listed....
Is the ceiling painted with...what? Distemper? Limewash? Scrim tape over the cracks, heavy coat of unibond and reskim the lot and live with the ridge.
Problem with retro-fitting battes is knowing where the old ones are..spacing was never that even...to fix the new ones to them.
Or glue/screw plasterboard - in varying layers - to remove most of the unevenness followed by a bonding coat to fill in the gaps then a reskim.
Without seeing it, it's hard to be more precise.