@jrfb said:.
My question: Should we have first of all asked them to remove the whole of the concrete since the work was illegal?
They bought the property not knowing of this issue and we weren’t aware of it until the dampness became a problem.
It’s a tricky one because they bought the thing in good faith but then they would or should have been aware that there were conditions attached to the exclusive use permission. I’d be as accommodating as possible with this one because your main issues is to prevent the damp. Make it easy for them to do the right thing and install drainage – and consider making a contribution on the grounds that there is no way the Owners Corp could not have realised the yard was being concreted without permission.
But if they want to get shirty about it, you could invoke the lack of the required permission plus a probable breach of council planning laws as an indication that if they want to play hard-ball, you could force them to dig it up at their own expense and start again.