Thanks Pete. You're the fifth or six person to recommend that! I love that we can take advantage of thousands of people's experience and get advise from woodworkers all over the world in a matter of hours.
I would have thought you would be able to sand the top flat on a board with sand paper stuck on it then a pass over the router would parallel the rebate. If you hold the newly flattened side against your router table fence with a rebate cutter fitted and pass the lid left to right between the cutter and fence and take light cuts till it is sorted.