DD did the SP exam last year, she said she have 1-2 questions not sure in maths section C, but last year's maths was not really difficult according to her. Anyway I think it is less about the absolute mark but about comparison against the peers who took the exam at the same day across three papers (heard they have some internal weightings), as well as other non-academic criteria. The school aim to cut the number by half for interview.
As for CLSG interview, last year is 15 minutes, well structured, firstly some questions on 2 picture shown for comparison, then some general questions to find out your DDs interest etc.
Good luck