I had to sort out a PAYG sim today for someone who will barely use the mobile but needs to have one for safety purposes.
Anyway, the Asda PAYG was best. 4p per min/text/mb. Must make a chargeable use every 6 months to keep SIM active. £5 min top-up.
I reckon you ought to be able to set up a punt with 4 texts and 2 calls, ie 24p. So £5 credit should cover 20ish punts. If you're an infrequent punter and top-up £5 today you just need to remember to send a text somewhere in June to keep credit alive. You'll still have £4.96 at end of December.
You can buy a SIM for 50p cash in Asda, register it to a burner email online and top it up with a voucher bought for cash. All totally anonymous.
In terms of length credit stays active and low cost of use I believe this is currently the best all round PAYG SIM at the mo'.