I remember seeing people talking about how many people get their grades changed by the exam boards and I would like to say that generally the exam boards do a pretty good job, although there have been a few scandals I think where there have been major mistakes.
I come to this conclusion due to the fact that the majority of those who remark don't get their grades changed and those who remark generally believe their grades are wrong so the probability of their marks being wrong is higher and with such a small error in marking it usually comes down to human error,subject subjectivity in subjects like English/History and standard statistical variation as those Maths students who have studied Statistics know examiners can only try and make their process more accurate(99%,99.1% etc there will always be a decently wide margin of error as it is simply impossible to mark every paper correctly, think about how many times you have bought faulty goods and then you will realise how good of a job examiners do).
This does remind you how easily things can still go wrong on exam day, even if you really did do well and Universities have no legal obligation to keep your place until the remark comes back.