As no-one more knowledgeable has answered this, I'll have a stab.Many office workers in the City of London had to work on Saturday mornings during the postwar period and, although there was a gradual decline, the practice did continue into the 1970s, although had probably almost died out by the end of that decade. The Waterloo and City Line had no weekend operation except on Saturday, with the last train from Bank being at 2 p'm. ish. On the buses, you still had examples of routes with projections to e.g. Cannon Street on the 95 route for those Saturday workers. My father worked for the Bank of England and certainly had to go in on some Saturdays. Could this be the reason for additional services?