Further to my extender problems. Canon contacted me yesterday to say that they could not find a problem with the extender and offered a few options including sending the camera and lenses up to them. I declined that offer as I do not think mailing a 600mm at this time of year is the best option. They have returned the it to me with a note to say that If I have further problems to send them images and possibly take the extender and lens to them. I will run some more tests to see what it is like. Really the purpose of this post apart from updating as I promised is to say that the service from the Canon service centre has been excellent, they have cleaned and tested the extender and not made any charge. They even paid the postage both ways. For an item that was out of warranty I think this is really excellent. Sometimes we are quick to complain but I think good service should also be noted.
I will test the extender after Christmas and post results here really to close the loop on this thread.