Police force's (and the Crown Prosecution Service) are not obliged to stick to the guidelines of the Association of Chief Police Officers but most forces would do so unless there were exceptional circumstances (e.g. icey roads or speeding outside a school as the children left the premises).
Under the ACPO guidelines, speeding in a 50mph zone would normally only result in a summons if the speed of the vehicles was 76mph or higher. So you should be eligible for a fixed penalty of 3 points plus a £60 fine. (The option to accept that penalty, rather than go to court, should be on the NIP).
Points remain valid for totting up purposes for three years. However you can't have points removed from your licence until 4 years have expired. (There are some serious motoring offences where points must remain on a licence for 11 years, but they're not relevant to you).
Chris