After doing a fair bit of research regarding getting a better pair of phones for mixing, I settled for the Shure SRH840
Initial impressions are promising, much more detailed than my Sennheiser HD280s, a lot more comfortable (the HD280's have very strong clamping), but with slightly less isolation. I was going to get a pair of AKG K550s, but I tried them on in a local hifi shop and I couldn't get a decent seal with them (on my rather large head !) , so I couldn't get a decent bass response.
From what I've read on various forums, you can go a bit mad with the eternal hunt for the 'best', so I went for something affordable that will hopefully do the job for a few years to come.