Another one who held out against the mobile...I satute you.
I broke down about a year ago, and I know how you're feeling. But, I went into the shop, said I want the most basic pay-as-you-go I can get and they showed me three options. Easy as that. It was in a studenty area, so maybe they're used to us wanting cheap plans.
If they aren't as nice to you as they were to me, ask them to write down the details of their offers and go somewhere else. Or do some online homework first. Don't worry, if you come in wanting pay-as-you-go, there isn't too much they can force on you (except a really pricey phone)
An interesting side note-I still haven't used up the ten pounds I bought with the phone...I'm on orange.