Complex sentence: Staff moved in to the building on the weekend of 12-13 July 1975, and it was fully operational by September 1975 and was officially inaugurated as the headquarters of BBC North on 18 June 1976.
Split into simpler sentences: Staff moved in on the weekend of 12-13 July 1975, with the whole of the headquarters in operation by September 1975. The centre was officially inaugurated as the headquarters of BBC North on 18 June 1976.