I'd suggest spending more time on subjects that you are weaker on, but the time spent will probably vary week by week.
Trying to consistently study is probably more useful than doing massive long hours sproadically.
I'd suggest ringing BOSTE (i.e reporting it if possible).
Does your child expect to perform better in externals? Not sure if it will be possible, but might be worth checking to see if it may be possible to apply for misadventure and only have externals count?