Thanks, Chris. I notice that I have some dust on the sensor, seems to be more evident when I am going for no dof, at f16. As this image hasn't see Photoshop yet, I must remember to clone out the offending dust spots.
Since I took this I bought a simple rocket blower, took off my lens, turned my camera so that the hole pointed towards the floor and gave it a few puffs. Seems to have worked.