Apparently your child can take the 12 plus exam in their first year of secondary school. This link might help.http://www.bexley.gov.uk/index.aspx?articleid=4297
It says: Children can be admitted to grammar schools after the start of Year 7 if vacancies arise because, for example, other children leave the school. If you would like to apply for a place at a grammar school for your child, you should complete your home local authority's Secondary In Year Admission application form listing your preferred schools and return it to your home local authority. You can apply for schools in any area on the same form.