Thanks all for your comments & input on this one. I'm with Nigel in that I expect that when I pay serious money for a pro spec lens I expect it to focus where I want and certainly require no more than 1 stop down to get the best.
It was encouraging to hear of other members being extreemly happy with their L series glass but after a week of experimenting with real life situations, I decided that this particular sample was not for me and sent it back. I also decided that I missed the IS of my 17-85 and the 70mm long end was also limiting for the kind of pictures I tend to take.
So.... I decided that the 24-105 f4 IS L series would be the one for me but I had read on some forums that there could be sample to sample variations with this as well. I decided I wanted the ability to easily exchange it if I was not happy so I went to Jessops (shock - horror) They were offering this lens at £680 but when I said that I had seen it in Bristol Camers for £639 (warehouseexpress £649) they agreed to match that and I got 12 months interest free credit
When I got it home and checked it out, I still didn't think that, at short focusing distances, it was as sharp as it should be
I tried the "photographing a peice of newspaper at an angle test" and sure enough, the sharpest point was about 30/40mm behind the focus point. Again on landscapes it was excellent but I guess the DoF would take care of a small focusing error at infinity. Anyway I took it back and Jessops exchanged it no problem.
And..... this time the lens is super sharp even wide open edge to edge at all ranges
Clearly Canon have some quality control issues and the moral is to test your lens at a range of setting to make sure you are getting what you pay for.