How much extra-curricular information do Oxbridge want to see on my personal statement?

I'd advise keeping extra-curricular activity to a single paragraph of your statement, towards the end. When it comes to Oxbridge, interest in your subject and a desire to learn are key. Of course, you should be able demonstrate some sort of activity outside of academia, but these two universities are ultimately interested in your academic potential. If you are running out of space on your statement, exploring a subject specific idea that really means something to you should take precedence over the fact that you have a DofE award. Remember, at interview, they are likely to ask you about what you have written on your statement!