I don't know exactly when they started to dissappear. In the mid 80s companies like Unigate saw what was happening to the business and started to push "partnerships" where the milkman would not longer be an employee with a salary. The milkman became a salesman on commission and I believe that was the beginning of the end. In the mean time Unigate, like other companies continue to develop the business of selling milk in large plastic containers to supermarkets etc.
I think (at a guess) we had a milkman coming every other day until about 7 years ago. We used to have a small order just to try to keep the guy in business but, like most family, we used to get most of our dairy products from the supermarket.
A sad end when I think of it.
I remember when people used to deliver coal for the fire....