I think according to the relevant Southend on Sea Secondary Admissions booklet, SHSG initially took 7 on appeal so they actually only had 6 places physically unfilled as at 1st September 2011. One of the current year 9 girls told me that 'about 18' applicants took the in-year test (sitting the same maths test as they did for setting), which would tally with an actual figure of 15. She seemed to think that only one girl got in on the test? I think someone posting on here said that their DD transferred from Westcliff after only a few days (I may be mis-remembering this!), which would have left the year 4 'light'.
The following year started over numbers due to appeals, I think, but there has been on-going movement. Our DD and 2 others were admitted right at the end of year 7 via an in-year test last June, and another girl (also from DD's first secondary school) joined the same year via this route last term.
So basically don't give up hope if you don't get what you were hoping for initially, because people do move around