plasterboard \cladding

[gg1a]

i am installing decorative cladding panels on ceiling but previously had leak and plasterer put a new board up but its sags a bit , therefore battens only touch in parts ,whats my options? do i remove all the plasterboard , and fix cladding to existing joists?if i cut bad bit out then will have same problem of batten only hitting good plaster and gap where old bit was i will have gap between batton and joist.
need help fast
ty

 

[noseall]

surely, if you are fixing battens into the existing joists, there will be enough pull in the fixings to straighten out any plasterboard dips?