Lens Calibration (Canon) - where and how

I have a Canon 24-70 L that seems to keep bieng out of focus - I know it's not my 1Ds mk2 cos that just came back from being re calibrated, - whats the best way to check the focusing accuratly and where should I send it = does Lehmans do this type of work?
cheers
Paul

Set the camera up on a tripod and a metal ruler with clear markings on a table. The camera should be pointing at 45 degrees to the ruler.

Focus on a set point with the lens set at f/2.8 (or widest aperture if checking other lenses)

The resultant image should tell you how far out any focus is. Do it with a couple of different lenses if you can to get an idea wether it is the camera or the lens.

HIH

Ian