Good morning Stewart,
Last month I picked-up the Sigma 20/1.8 from Jessops for £185. I went with it, rather than the Nikon 20/2.8 as I wanted the faster lens. I never got to use it until this past weekend, and I have to say I am very pleased with the results.
On FX (my D700) it is really wide, quick to focus and I love the short-close-focusing distance. It is, mind you, a rather big lens taking 82mm filters.