In answer to a few comments.....
There is a certain amount of assuming that if you have a baby, you are entitled to a three / four day week and an assumption that employers will just have to work round it.
Unfortunately, the legislation forces employees into direct confrontation with the employer rather than reasoned negogiation which has significant downsides as it's difficult to talk through the options and come to the best scenario for everyone without employer / employee feeling railroaded one way or the other.
In my rose tinted world, "flexible" would mean more than it does now, allowing a range of options, not constrained by the number of days worked. Job roles would be more flexible and the focus would be on getting the job done.
There is a need for greater appreciation of the issues from both sides. Employers should recognise that motivated part time workers may well do everything more efficiently, better, faster, cheaper (and feel short changed). Also employers benefit from the fact that you do not stop thinking about work when you are not there.
Employees should recognise the impact on employers / fellow employees. The little things not in your job description but keep the business running smoothly are not as easy to do if you work part time and it is more difficult to react to short term changes in work load. An extra two hours on a full time week is not much, but on a three day week, constrained by drops offs and pick ups is!
Hopefully as more people work flexibly, we will get better at it!!!