Was it delivered by recorded delivery? If not then...what letter?
The law doesn't work like that. It assumes that any letter sent by standard first class Royal Mail is received within two working days, regardless of whether it got lost in the post or not. Try ignoring a NIP for speeding using that little line, see how far it gets you.

Guessing the law company representing them is ACS Law...
Link reading fail. It's Gallant Macmillan doing the letter writing this time.