tester wrote:
Hi,

I appreciate you can level individual batterns horizontally /
vertically using a spirit level or similar, but if the walls are bumpy
and irregular, how do you level the whole support across large spans
so that the plasterboard sits perfectly across the length and breadth
of the wall?

Do you fit spacers or something - I've been pondering the best method
but if there's a correct way to carry this out?

It's easier to level everything with a skim coat of plaster. You can
pack battens with bits of hardboard or whatever but it's a painstaking
job. Even more fiddly if you're fixing to the original joists