Tories love to taunt the Labour Party by saying that it is in hock to the Trade Unions. A more complicated version of this is to say that Labour is a wholly-owned subsidiary of the Unions. But the people making these taunts know that they are not true. It is true that about 87% of Labour's funding does come from the Trade Unions (source: http://www.guardian.co.uk/politics/2011/may/25/labour-party-donors-unions). But Ed Miliband refused to back the strike action taken by four Unions in June this year, and reiterated that refusal in his speech to the TUC Congress yesterday, a position that was greeted by boos and heckling.
So given that the Unions are overwhelmingly Labour's main source of funding, why won't the leader of that party do exactly what they want, and why do they put up with it and go on funding the Party ?
The answers come if you consider their alternative. They are free to withdraw funding and therefore bankrupt and destroy Labour, and set up their own new Party whose policies would be under their complete control. The problem is, no one would vote for it, and they know this. Labour may not give the Unions everything they want, but they are a viable political party with a network of activists, historical roots and a base of support in the country, which can and have been translated into becoming the Government. Ed Miliband, or any Labour leader, may appear to hold a weak hand because of Labour's financial dependence on the Unions; but on the other hand he knows, and the Union leaders know, that the Labour Party is the best offer they are ever going to get in terms of meaningful access to power, and that effectively the Unions have nowhere else to go.
This explains what is otherwise a puzzle: why the Unions would go on supporting a Labour leader they were primarily responsible for installing who does not support them on strikes over what they regard as critical issues for their membership; and why they would go on funding the Party of which he is a head.