John McDonnell is proposing that the working week be dropped to four days. This assumes that there is a fundamental imbalance between employees and employers which is not, presumably, corrected by the existence of trade unions.
It's an interesting idea, but it is one more assault on the ability of autonomous citizens to enter into contractual relations with one another and therefore a further notch down in our ability to maintain any sort of growth in productivity.
I don't suppose that in practice this will make much difference. The working time directive of the European Social Chapter does not prevent investment bankers working a hundred hours a week. Why should this be any different. It's not as though someone wanting to work longer is going to sue.