Hi Mendonca
I can see that things are worrying but there are a few factors that it is worth bearing in mind and some work you can start doing....

- the catchments will increase a little from current figures as a few places are not taken up (by people moving away or going to private school)
- the birth rate rise may have peaked and you may be past the worst (I have heard that we may be over the peak but have to admit I don't know how this relates to your sons age?). This may mean there is a little less demand???
- the slow down in the housing Market may mean that less young families are moving into the area (a bit of a long shot!)
- local councils are planning for permanent increases in school sizes in areas of most demand. Certainly your area fits into this category so find out what Lewisham and Southwark are planning. Ideally once this years intake has settled down in a few months.
- Speak to the Council admissions departments. Explain your concerns and ask their advice. It is harder because you have two authorities to deal with.
- find other parents in a similar situation close by and consider approaching local Councillors to raise the particular and real problem in their consciousness
- look at schools in a slightly wider circle, just so you know what other choices you might have

Above all good luck.

Good luck