WOAH! Just to quickly voice my opinion before I dash off to London:
I think if you have the option of doing further maths then make sure you do, but if you dont then don't panic and don't let that put you off from applying! My school only allowed us to take Further Maths as an additional 5th subject in lower sixth and this meant studying it in just 3hrs a week over 2 double periods which should have been my frees. I chose not to take this option as I thought I would lose out on valuable time reading around my subject and making sure my Physics and Maths were both as good as they could be and when I explained this to the interviewer at St Hilda's (I was not questioned at New regarding this) she seemed satisfied with my reasoning. Luckily I'm going to be trying to learn P4-6, or at least highlights of them, over the next few weeks before Uni starts.