I work for Royal mail mate and i can tell you that they/we would never ask you for your DOB.....was it a recorded or special delivery item..? only reason i ask is because we would only use a PDA for getting signatures.....
to me smells very funny...best thing is to go and collect it and see what it is safest bet ....
Also the red delivery cards would have an option to pick up down your local royal mail office...