There is a graph showing the distribution of marks that is sent to parents appealing. The size of the cohort is around 7,000 and of those aproximately 250 will achieve 141. (These figures are very rough indeed!)
SVE is quite right - catchment is the first main criterion for allocation of places. In 2006 AHS offered places to girls up to 15 miles outside catchment. In 2007 every applicant was offered a place regardless of catchment and they still had 37 vacant places in September.
You should be set fair, "based on past performance". It could be different this year, but I doubt the difference would be sufficient to deny you a place if you are in catchment, at least