tethersend - I completely understand the importance of employment legislation. However, I think the broader issue here (which wasn't what the OP raised but which has kind of grown out of the thread!) is where the legislation could potentially backfire.
I think there is a huge difference between people who work part time because that is what their employer wants them to do, and people who work part time because it suits them. Most people in my school who are part time have actively sought to become part time teachers. It is a decision driven by them, not the employer (although of course the employer has to agree it). It's a tough call, because an employer has the right to turn down flexible working requests quite justifiably for a number of reasons - eg if it costs the employer more, or impacts unfavourably on clients or colleagues. If you have a situation where someone two people share a job, one working two days and one working three days, and the school ends up paying one person to come in for vital things on their day off - eg essential training, or a parent consultation, then their part time status is directly costing the school more. Likewise, if someone works 4 days a week and another teacher has to pick up the odd lesson's teaching, then it is impacting unfavourably on the colleague and the pupils. (I know someone earlier painted a rosy picture of filling in for the occasional lesson, but let's be honest here, it is rarely like that. Split classes can be horrendous, with no proper sense of ownership for the person teaching the occasional lessons, and a lack of continuity etc).
My point is simply that in situations where the employee wants to not work full time, then it's in their own interests to make it viable, because the upshot of problems is that employers will be less likely to agree to part time requests.
tethersend - you said earlier that I am looking at it from the employers point of view, whereas you are seeing it from the emplyees. I don't entirely agree. Yes, I am senior management, so I do see the whole school perspective, but I am also an employee and a mother, and actually I think this is about trying to see it from every angle, not least the pupils'. The school is there ultimately for the sake of the pupils, not to provide convenient jobs to suit the wishes of every individual. In reality, my school works very hard to try to be fair and reasonable, and we always consider requests for part time working through the proper channels - and actually give considerable time over and above to do this! I have sat in meetings until late in the evening, thereby not being with my own children, to consider the requests made by staff to reduce their working hours. This shouldn't be a 'them and us' mentality, it's about everybody treating eachother reasonably. Most of the staff where I work are parents - it's not just the part time minority. We have 96 teachers in my school, of whom 68 teach fulltime. As far as I am aware, around 70 of those teachers are parents. At the end of the day we have to be fair to everyone.