The rear screen demister stopped working the other day, so after checking the fuse (OK) I opened the roof to a half-way position and looked at the connections to the element, to find that even though the wire was fitted to the spade connector - the copper tag itself had become dislodged from the pad on the element.
The next thing to do was to remove the cover, which was a bit fiddly but not too difficult. Having good access to the area, I removed tarnish from the tag and the pad on the element with a small file to expose the copper. I used a gas soldering iron to tin the tag and the pad and then using a pliers to hold it, I soldered the tag back on to the pad. I closed the roof and started the engine to test the demister . . . and it worked. I then opened the roof half-way again to re-fit the cover.
I hope this post helps anyone who has a cc with similar issues.
One word of warning though - if you have the roof partially opened/closed, I think after around five minutes, the car beeps furiously, a message appears on the display and the roof opens/closes/continues on its way automatically. 
So you need to bear this in mind if you're doing any work that involves having your fingers or tools etc between the levers and support struts of the mechanism.Here's a pic showing the re-soldered copper tag:
Note: the black stuff on the female spade connector isn't evidence of heat or burning (as I initially thought), it's some sort of insulation compound.