stonechat

taken today whilst looking for red deer on the quantock hills, somerset canon 40d Canon EF 100-400mm f4.5-5.6 L IS USM Lens 1/250 f5.6 ISO 400 hand held

It looks quite out of focus. I'd suspect camera shake is the likely culprite here. IMHO you really need to be using a tripod, or a monopod at the very least, to get the best out of that lens.

ill bet its the lens, i had one and never got a great result out of it im sorry to say, but as justin says use a tripod youll find this even with the prime lenses, well done for getting so close to this one, did you find any deer?

ben/ yes i did find some deer but did not get near to them to many people about will try tomorrow morning first light will take my monopod/ mike

Strange plumage, the pale throat indicates an autumn plumaged female

If IS is enabled are you giving it time to spin up? - you need about 1 second from half pressing shutter release for IS to get up to speed and work properly. There again the 100-400 is notorious for soft sample variation

Never had a problem with the 100-400 myself, having always found mine to be dead sharp when I was using it properly, which I admit took me some time. Mike's point about the IS is a good one and is Justins about the tripod and both will improve your results. I believe that you are not supposed to use the IS when the camera is on a tripod, but I've never found that doing so made things any worse.
I think your shot could do with a little more saturation and some noise removal from the background at full size; and some more usm after reducing in size for the web. I've tried it and it perks up nicely. More left in this shot IMO.
Eric