Not sure but as there are about 26 kids in my dd's class and one teacher and one assistant it must be 1-13 but there could be 30 in the class and there would be no more teachers so that would be 1-15 (hey I can do sums!!). 1-12 sounds good to me!
When DS2 was in reception the class size was 40 () but they had 2 classrooms, 1 FT teacher, 2 PT teachers and 2 PT nursery nurses, so there were always at least 3:40 which is slightly over 1:12 but close.
Also there tended to be sundry other voluntary bods about at various times - parent helpers or work experience/PGCE students - so often there were more than 3 adults and the ratio was better.
22 kids in class, 1 teacher, 2 teaching assistants and frequently 1 student teacher and 1 teaching assistant student. our reception teacher does a fab job of recruiting! (helps that we have a teacher training college nearby)