The first NIP has to be issued to the registered Keeper within 14 days, but there is no prescribed time-limit for second and subsequent issues. If the original owner took all 28 days (which does the real culprit a big favour, towards hitting the 6 month time-out) then replies saying he sold the car to a dealer, the cops issue a new NIP to them. They then return it and the cops send you this one.

But you have 28 days to return the NIP - hang out to the very end and you are over five months into the 6 month cut-off , then write back saying you have checked your diary, learned this is the day you bought the car and have no recollection of having sped at all - then ask for pics to aid in identifying who was driving at the time (you don't really want these, but it is just to stall the thing for another 2-3 weeks.) I'd say you will be very unlucky to get done on this.

Don't just ignore it though, because there is an extra 28 days (I think) for the separate offence under S172 of failing to provide info and that carries a much bigger penalty including jail-time - but no points though!