Tesco Mobile are now giving you TRIPLE credit on £10+ top up on PAYG.
e.g. You pay £10, you get £30 credit for the month, pay £15 get £45, etc.
They also have a £2/week unlimited (by unlimited they mean 100Mb per month!) which you can set-up to come off your credit every Monday. So in theory for £10 a month you get unlimited data and £22 credit for calls & txt.
Any unused credit that you got free is removed at the end of the month but you keep any paid credit you have. Free credit is used up first.
Seems like a good deal to me for PAYG and you want a data package.
Flat rate is 20p a call and 10p a txt but you can set up 5 fav numbers and they're charged 10p a call and 5p a txt.
I think Tesco use O2 as the network provider...
You could maybe get £1 Quidco for ordering the sim too.