Turns out it's less baffling than I thought, because I can't actually get Working Tax Credits, being under 25 and working under 30 hours a week. Basically, if you're under 25 then you should be at home with mummy and daddy, and not really doing part time work because you might get too many hours which would mean losing the few benefits they do give you.

I don't mind losing my JSA, because it gets cut down if I work and goes back up if I don't, so if I'm working it's fine. The only problem is that if I'm not on JSA, we have to pay the full council tax, which is considerably more than I'm gaining in my three extra hours.

Oh, and my agency have me on the wrong tax code.