Mini Display port problems

I have jusrt received my 27inch LED cinema display and have connected it through minidisplayport to my ATI 6950. Unfunately for some reason the only available resoltion appears to be 720p and I get a warning when connecting the monitor about there being a problem with my mini display port connection and therefore I will not be able to access all the monitors possible resolutions. I am currently updating all my drivers etc to see if that allieviates the problem but otherwise I'm a little stuck.

Has anyone else had a similar problem and know of a solution?