Actually, sp, sarah has to prove nothing. It is for the prosecution to prove the matter beyond reasonable doubt. In this particular case they will have to produce evidence to show that the speed limit was 30mph at the location in question and that the driver was exceeding that limit. If they do both those things I don't think sarah will have much of a defence.
If she is acquitted she cannot claim her legal costs but the Bench may award her reasonable travel expenses.