Not too sure about how long this type of record is kept for, when I worked for Post Office (RM) 3-5 years was the maximum time depending on the type of transaction.
But you could contact the archives at Pheonix House (back of Mount Pleasant mail centre). Just google Post Office Archives or Royal Mail Archives and the address and phone number should come up. They are used to us family historians contacting them. And if you lived within reasonable travelling distance the archives open some Saturdays.