davewhiteside,
I suffered from a progressive early/mid sixties education which left me really weak on mental arithmetic. Though I had the required pass at C I still was very worried prior to Biggin Hill.
To prepare me for OASC I spent some months with a wisened old Halifax navigator who took me through endless speed/distance/ time calculations, initially on paper and then mentally. This seemed impossible at first but after constant and seemingly endless repetition I came good. Once it clicks it's like learning to ride a bike, for ages you are hopeless and then all of a sudden it clicks.
It got me through OASC but, and this is a huge but, it also helped me through what was then a horrendous thing called Academic Training Squadron at South Cerney and primary and basic flying training. Without it I would have undoubtedly been chopped either at primary or Basic.