SAQ Cambridge Watch
The additional PS in the SAQ is entirely optional. You should normally only write one if the course at Cambridge is in some way different to your other choices.
"7.1 Optional additional personal statement
We will already have received a copy of your UCAS personal statement. If you wish to add anything specific about your application to Cambridge please do so in the space provided. We would be particularly interested to know what aspects of the Cambridge course attracted you to apply here. Your answer can be a maximum of 1,200 characters including spaces. We strongly recommend that you type your personal statement directly into the text box (rather than copying and pasting) as some characters are not supported by our system, and this may cause problems with the submission of your SAQ.
Not providing an additional personal statement will not disadvantage your application in any way."
PS. I've moved this to the University of Cambridge forum.