I don't understand the bold part, please tell me about it? Perhaps there is a retired plod with a 50-year-old duty roster that can give us a more definitive idea of how officers then spent their time.
(Oxford Living Dictionaries)
A duty roster is "a list of ... personnel [police officers, in this case], specifically one showing their regular assignments and periods of duty."
The writer apparently believes that 50 years ago police officers in the UK performed different duties than they do today.