Notice of intended prosecution MUST be recieved within 14 days of the alleged offence or it is unenforceable.
When the notice is recieved within 14 days though, Best way out of them is to write back asking for a photo of the alleged offence, (legally, they must supply you anything that you ask for if they intend using it as evidence) and then, presuming the image does not identify you, write back explaining that you have narrowed it down to two people- yourself and one other person - then give them full details of both persons and ask them how best to proceed as you "wish to be as cooperative as possible".
It is not possible to prosecute two people for one offence in this situation as only one of you could have been driving, they cant prove anything either wayand you are being seemingly very cooperative, hence a court case will simply not happen - case dropped!
Has worked for me and friendsof mine 13 times in a row!
Good luck