Put the lens on tripod, set ISO100, f/8-11, and shoot some house or building & see what happen.
I still wasn't happy - so took it in : appears that it has a faulty AF motor.
As its still in its 3 month warranty I am talking to the shop; if they will pay for the repair (£250) I will keep the lens.
Its not something that was very obvious until I started to try and shoot moving objects- it was seemed fine on static objects.
Just my luck - the lenses are hard to come by used and I certainly can't afford a new one - this was at the top of my limit as it was.
Edit: Looking back at some of the other pictures here with this lens, all of mine were soft in comparison!