So the school now has sent me the 2015 grade boundaries:
For a C, B, A, A*. Lower than C is only D
Maths: 50, 63, 75, 85
English: 40, 47, 55, 66
Science: 50, 57, 70, 80
French: 55, 65, 76, 90
History: 45, 50, 65, 78
RS: 55, 65, 72, 90
Geography: 50, 60, 70, 85
Latin: 45, 51, 70, 80
Spanish: 50, 65, 70, 90
Greek: 55, 70, 85, 95
So you can get all C's with a 49% average, all B's with 58%, A's with 69% average, and then A* with 82% average.
So the 65% passing grade is like a B+ average, and DS could have got anywhere between 73% and 88% average, perhaps he got around 80% overall.
In terms of %, the raw grades for English and History are not particularly high, thought he got an A* for History, so he did get a good raw grade there. His A in English only translates to 55-65% so that would be the only subject he got less than 70% in, but then again you can't really compare with other schools because English marking must be very subjective, the same paper could get 60% one place and 75% somewhere else.