You have 28 days to furnish the info. There should be a telephone number on their somewhere for the traffic criminal justice unit. Personally I'd either call or write to ask for more information regarding the alleged offence as you'd still have time to send that letter back in.
Personally I've never known blanket NIP's. Only if evidence exists, but I'm not saying it doesn't happen.
I've known sec.59 notices to be abused and they shouldn't be.