I've just stripped and rebuilt my rear XT hub and now I cannot for the life of me get it to rotate freely (or even close to freely) without having play in it, as soon as I adjust the play out it will barely go round at all.
Any pointers? the cup+cone faces all look fine bearings are all shiny and round looking – the wheel did go from being fine at the start of a ride to being very loose at the end which is why I ended up stripping it down.

This has good info on adjusting C&C hubs:

http://sheldonbrown.com/cone-adjustment.html

I know (the theory of) how to adjust them (I've only ever done one before – but that was pretty easy), just in this case the point at which the excessive play starts to appear is before the wheel rotates freely enough.
I haven't got to the 'it feels alright before doing up the QR' bit yet…

Are there any other things which can cause an issue other than adjustment?

Take it apart and check that your freehub body's tight in the hub. If it isn't then you'll never get it adjusted correctly, damage your hubshell and probably have the bolt work loose 7 miles from home and seize the hub solid, chewing up your dropouts in the process(surprisingly quickly too, a matter of seconds thinking somethings wrong and it's eaten a lot of metal).

Fixed – turned out I'd stuck the washer that goes on top of the balls on the drive side upside down so the cone was tightening up on that rather than the balls themselves.