I must reiterate the problem I had which was the myriad tiny oil spots on the sensor, measured by FoCal as over 400 teeny ones. That has to impact on the overall clarity. There were also a few bigger specs of dust or something more solid.
I tried wet swabbing the sensor and although it cleaned it up slightly, it didn't shift the oil, so I sent it back to Canon Elstree to do the recall work and to clean the sensor. They reported the sensor had been scratched (this was presumably with the dust/grit trapped in the oil) and it has cost me £600 for a new sensor.
I'm not sure what lessons there are in this cautionary tale other than I don't think I shall be cleaning my sensor again. I've not had a problem before on other cameras but that is far too high a price to pay, and I don't think I can press Canon into refunding me the cost, as they deny there is an issue with oil spattering. And yes I do use the 12fps quite a lot btw
I will check again with FoCal on the stte of the sensor now that it's been modded and replaced and returned, although I can't find the black mark in the battery compartment. There is no mark on the silver plate with the slot in it