Thanks guys.
It is a brand new monitor. I'm using the supplied cable miniDP to DP. I'll try the DVI cable to see if that's better.
The rMBP thread is interesting. However, I'm pretty sure that my monitor is already using RGB. When I set the monitor to YPbPr then things get very weird.
I assumed that out-of-the-box the Dell would have 'good enough' colour consistency. I bought it to replace two 23" Apple displays which run so hot that they are basically radiators which also happen to emit light - which makes my home office unbearable in the Summer (high thirties Celsius).
I was torn between buying the Dell in order to eke out another couple of years of life from my 2009 Mac Pro or buying a 27" iMac + thunderbolt storage array to replace it.
Right now I'm leaning towards sending the Dell back and buying an iMac. Expensive, but I'm confident that it would work.
Best Regards,
Julian.