Thanks everyone for sharing your thoughts on this.
He put the new plaster board onto the old plastered wall. I guess I assumed he'd be working to get things as plumb as possible but it's too late to do anything about it now. He's not the type of person to accept any responsibility for a mistake!
Colour Republic, thanks for suggesting getting the panel cut bespoke - I've suggested this to him but he's not happy about having to take responsibility for measuring, he's talking about angles having to be woked out. I guess I'll have to measure it myself, it's not an exact science is it, we're just trying to remove some of the variation in width over a 2m height? I thought if I drop a plumbline at the width point and measure to the wall in a few places from there I'll get the width at the bottom and top of the panel. Does that sound ok?
Thanks again.