Tesco mobile extra tarrif is 10p calls any network and 5p texts, You have to topup with £15 to be on the extra tarrif but depending how much you use your mobile this isnt a problem.
If you have or can get virgin media they do the 3 for £30 or 4 for £40, meaning tv channels broadband landline and a mobile sim card, the sim card has 300 mins and 300 txts per month. You have to have it as part of a virgin media package but works out good value imo. Its not exactly payg but its not a contract either because you can cancel the sim anytime you want after the first month as far as i am aware.