Well to be honest I would list every possible grade that is on my name.
I don't want to hamper my chances by being dishonest. It doesn't give a good impression to the university guys.
Also I'd suggest you retake that and improve. A lot of people including myself weren't great at maths but we've always had that liking and a bit of ability so keep at it.
As regards to uni, is there anything in your mind that is making you say that Cambridge is the be all and end all?
If not then there are many other universities which will at the end of the day give you a degree which will in turn open doors for you into a good career.
Don't get your heart too set on Cambridge, it will be of no benefit to you.