Thanks for all the replies.
I took it to my LBS and he agreed that it wasn't right. He inspected the hub for play and it was fine.
He added some spacers from a 10 spreed road cassette. This reduced the wobble but didn't eliminate it. Tried a new cassette. No improvement.
As I was rushing off for the weekend I took the wheel with my old cassette and the spacers. The first time I tried to pedal it jumped all over, but it was soon fixed with a few turns on the barrel. Rode the bike hard over the weekend without any problems. :-D
Also, after I mentioned the Mavic extra-wide hub feature, he had a look in the Mavic catalogue and found a spacer that fits the grooves on the hub so hopefully, even the remaining wobble will be eliminated.
Cheers,
Andrew