Just given the bike a good clean and when I've put the back wheel back on, the gears are skipping, so after a quick mess I noticed that the cassette wobbles as though its buckled when I spin the back wheel. Even when the cassette isn't turning and the wheel is, its wobbling around. Any ideas as to what it could be?

knackered freehub? What's the hub?

what hub/cassette is it?

http://www.singletrackworld.com/forum/topic/cassette-moves-on-hub

http://www.singletrackworld.com/forum/topic/cassette-wobble

Either a loose freehub or you missed one of the rings or spacers of your cassette I'd guess.

The hub is shimano deore and the cassette is shimano, I've had it since new and I'm pretty sure it didn't do that when I got it so doubt its a missed spacer.

TBH if you read teh threads shim freehub bodys wobble – my mate has an XT hub and wobble quite a bit but does not seem to be causing a problem 🙂

I suspect the axle bends a bit and give rise to freehub body wobble 🙂

Thanks for the links, just read the posts, I'll just try and sort the gears and leave it alone! If I can't do it then I'll take it to the lbs for them to a a quick look at, see what they think.

My Deore wheel has this too. Bike hung up, spin the wheel and you can see the rear cassette move about 1mm left to right. Im Guessing there is a slight bend in the axle or the hub flange isn't 100 perperndicular to the axle. Doesnt seem to affect it in use though. I think i've had this with other Shimano wheels in the past, hence why i have hope these days!

Yeah that's exactly what happens, enough people seem to have had this with no real effect on performance, good enough for me!