Appreciate this doesn't really add anything (as the question has already been answered), but if you were on GiffGaff, you wouldn't have to worry about data charges, because there are none. You get truly unlimited bandwidth for £10 a month. If you order a SIM through an associate link (like the one I used >
http://giffgaff.com/orders/affiliate/rhort ) then the SIM comes with £5 of credit on it. You also get (truly) unlimited texts and 250 minutes, and it's not even a contract; just pay a tenner and get the next month on those terms. They have other deals too, up to something like 1500 minutes for £25 (I think). Give them a whirl and see what you think of them.