"Knock yourself out. And if you smash the lens in the process, I'll not be that bothered."
Well after 10mins and some initial difficulties of a Canon user using a Nikon user's kit he'd come up with some very impressive pictures and notes to go with them. Essentially he was looking for the "Sweet Spot". This is the focal length and aperture that gives you the best picture. I know that the sweet spot on my Nikon 85mm f1.8 is about f4. However that lens has no zoom so f4 is always going to be the sweet spot.
The Sigma, however, has a focal length of 70mm to 200mm and so the sweet spot (or sweet "aperture") will vary depending on the focal length. He discovered the following ,very usable, results...
So while I was right that this lens is not useable at f2.8 most of the time I was amazed to see that at 105mm it gave really great results wide open at f2.8.
I now intent to keep the lens (at least for the time being) and to some extent it feels like I've been given a new lens. Just goes to show how far a bit of time, patience and experience goes.