I have had only one experience of having the sensor cleaned, on a Canon 400d in London a few years ago. Unfortunately, they were unable to clean it as the dust was too 'gunky' and wouldn't come off. They certainly appeared a very professional outfit, and though they didn't charge me anything, it was pretty disappointing. Maybe I left it too long before getting it looked at? Ended up giving it to a friends son who was studying photography at school, and bought myself a new camera.