Tuesday and Weds after 9pm delivery slots from Tesco are the cheapest... £3.75 I think. I usually order enough for 2 weeks, so it's not that much extra.
Edit: Also, my mother's just told me she got a letter about joining a Durham parent's club thing. Wtf is that all about?! I can imagine the type of parents who would want to do that kind of thing... *shudder*