Tesco mobile are now doing family perks. So if you have at least two SIMS on your account (I have mine and my wifes) you can choose a top up for free.
I've got for £12.50, 500mins, 5000 txt and 500Mb so can chose a perk of an extra 150 mins, 250Mb or 500 mobile mins.
I'll prob go for the data, but the wife will prob go for the mobile mins.
It's a one month contract and they use O2.
Not sure if you get bigger perks if you are on a more expensive contract.
As an aside. The customer services and setup has so far being really good.
You can also register a clubcard and get points every month depending how much your bill is. So i'll get points on at least £25 per month.
PLEASE DO NOT EXPIRE. THIS IS STILL A VALID DEAL.