The Labour Party has always been a broad church. Its members beliefs range the spectrum from neo-liberal, social democrats, Christian socialist, Fabians, to revolutionary socialists. Usually the balance of power in the party favours the centre or the right. In the eighties when the left tried to take control the party swung in the opposite direction. This move to the right was the furthest it had ever gone and probably peaked with the unopposed election of Gordon Brown as leader of parliamentary party. Now we have a Liberal opposition party to left of Labour.
For a long time power in the party resided in three sections, the parliamentary party, the constituency party and the trade unions. Trade unions tend not to be revolutionary as they serve a more defensive role in protecting workers rights. Two of these sections together could outvote the other. The unions and the parliamentary party usually held the party to the right or centre while the constituency party tended to be more to left.