After much vacillating I have bitten the bullet and gone for the Tokina 16-28, which I've ordered today. We're supposed to be going on a star shoot in a couple of weeks time - hopefully this rain will have disappeared and we will have nice clear skies. May have to contact John on how to create a filter holder to use it for landscapes.

I'm one of those females whose mechanical knowledge/ability is basically zero. I'm then going to order the Nikon 20 1.4 in a couple of months time. I really like the idea of it for a walkaround lens and also the fact that it takes 77mm filters which I have quite a few of.
Thanks for all the opinions everyone.