steve in leo, what are you like.LOL.
CC in most cases DOES offer a match or better MPG than relying on your right foot,its ok saying "a good driver"bla bla bla, but people still pull out on you or do silly things,you cannot anticipate everything no matter how good a driver you think you are.
and as I have drove 14 and 18 tonne with and without the CC on, fill ups don't lie.....for one thing modern diesels don't use fuel when the throttle is shut,and as we are talking about keeping a constant speed up, otherwise why mention CC, the CC will do this far better than you anticipating thinks after all, i know a fare few sections of m/way that look like you are driving on the flat, but are indeed on a long incline or vice versa, the CC will pick up on this quicker than you seeing your speed go down or up.
ps 40 mpg out of a 2.0l petrol is not as impressive as you may think, used to get that out of my r reg 2.0l mondeo on a run and more, with or without the CC.