tethersend - I completely understand your argument about the legislation - my argument is simply that the legislation does not resolve all the problems which part time working throws up.
The situation I described is a prime example. A 0.8 teacher, ie; one working 4 days a week, which is the majority of the time, could, in theory, never do a parents evening if it fell on their day off. Or if they do attend, they would be paid for several hours work each time, while someone working five days a week would get no extra pay? The legislation dictates that the part timer does not have to work pro rata - therefore, in theory, they can get away without having to do the equivalent, scaled down version of what a full timer does. And yet you said yourself that a part time job is 'part of the full time job'. Not a job which totally leaves out whole aspects such as parents evenings!
As I said earlier, I actually think parents evenings and INSET are the lesser problems, as usually in reality teachers see sense and realise it's in their best interests to do what needs doing professionally. I think split classes and the problems thrown up with timetabling are a far greater issue. Thankfully I have always managed to avoid writing the timetable in schools I've worked in, but yes, it's a nightmare, and one made far more complex by part time working which benefits the employee but not the school.
My point about economics is simple market forces. If the school is considering a part time post for the benefit of the teacher, then it really is in the best interests of the teacher to make it viable. Teaching is increasingly becoming an attractive option since the country has been in recession. My school regularly gets 30 or more applications for posts where a few years ago we'd have had maybe 5. There is soon going to be a situation where if a part timer is being inflexible, the school will simply appoint full timers, and I think this will backfire hugely on people who for whatever reason only want to work part time.
As I've reiterated several times, the situation where the employer wants someone to work only part time is quite different. The employee can then quite reasonably work in another job, or expect payment for anything over and above.