The Labour party was never controlled by union members. It was controlled by the officers of the union is the period from around the turn of the century when the union bureaucracy first established the Labour Representation Committee, 'till around the Macdonald government in the thirties. Then it became clear that the Westminster establishment could incorporate the Parliamentary Labour Party by a process of corruption and flattery.
After the war, where official labour had proved its loyalty by overseeing a system of super-exploitation, the trade union leadership was effectively incorporated itself into the British state system. From the period 1945-70 the union leadership was a conservative force in the Labour Party, as the 'block vote' was sued as a counterweight to grassroots members, especially the youth wing of the party, which was twice disaffiliated and suppressed with the support of the union bureaucracy.
In the seventies the breach between the parliamentary labour party and the union leadership came over the (then Labour) government's attempts to resolve the slump at the expense of the working class, threatening the union leaders' status.
After the Labour Party's defeat in 1979, union leaders' disappointed by the leadership declined to restrain the left of the party, who briefly imposed their own bizarre vision of a return to the 1945 government on the manifesto. After the electorate rubbished this exercise in left nostalgia, the project of modernising labour was quietly and slowly put into operation.
Initially, of course, this was a project started by the union leadership, as an extension of their craven policy of 'new realism'. But the unavoidable conclusion of making the party acceptable to Capital was to break its links with organised labour.
It's not well recorded, but the union leadership themselves supported this policy with a few sulks, but essentially it was them who made it happen. When NUM leader Arthur Scargill attempted to challenge the dropping of the party's commitment to nationalisation of private industry, it was union block votes that passed the Blairite 'modernisation'.
-- Jim heartfield