There's a space on the CAF for an 'Optional additional personal statement'. I take it that it's a good idea to put something here, but I cannot help feeling that whatever is put in the fairly small space provided is going to be rather trite.
It's difficult to say something additional to the UCAS statement and particular to Cambridge. 'Like the architecture', 'want to be taught by/ mingle with the best', 'will thrive in tutorials' all spring immediately to my, and presumably everyone else's, mind. Rather than simply prove that one has nothing useful to say here, I wonder whether it might in fact be better to be quiet?
How about: 'I am going to work for a first, and it may as well be one from Cambridge rather than a less prestigious university'? Sounds unbelievably horrific, as well perhaps as being unbelievable.
Perhaps play it safe and repeat some of the stuff from the UCAS statement, but in different words? How boring!