yep. He retired from Scotland Yard to make a more simpler life as a bus driver. But one of his proteges back from the Yard is now head of London Police and has to continually call on Manafort for his help. So he's always having to take his bus. Also, he uses his connections that he makes as a bus driver with his passengers to help solve the crimes.
Many episodes have a side story where Manafort has to help one of the passengers with an issue. Helping an abused child..or helping a store owner out of paying protection...that sort of things. Sometimes these stories are intertwined with the main plot.