As an update, I am slowly starting to gear my DK, he is now close to 4k GS and has cleared quite a few dungeons in Random Dungeons. The only place I had issues was in PoS where I missed aggro here and there on some mobs but the group overall was over 5k GS and so were a little more impatient with a poorly geared tank, although several times I did get compliments on the job I was doing with such a low GS.
Overall I may take a look at a frost tanking spec at the weekend once I have some more gear and maybe even dual spec him so that I can flip betweem the two specs. I do like the fact that as a tank he can still put out decent dps, this makes a nice change from some other tanking classes
Thanks for all the help