First thing: make sure there's a space after each punctuation mark. When typing in English, you need to physically add it. To answer your question, "the combined populations of Glasgow, Birmingham, Manchester, Leeds and Liverpool being added to the total national population over the two generations" is what we call a noun clause. It functions as an object. In this case, the usual verb becomes a gerund. How do we imagine 15 million extra people? By adding these city populations together. A equates to B.