Giffgaff are really cheap. They've had a couple of outages in the past year, but I don't think any more than other networks. Didn't affect me as I'm generally bathed in WiFi most of the time. They have different 'plans' that operate a bit like a contract but really it's just like an automatic top-up which you can cancel when you like. Or there's good PAYG rates too. I use the £10 monthly goodybag which has 500 mins to UK phones/mobiles, unlimited text and 1GB data per month and free calls to other giffgaff numbers. If she really doesn't use much data, you could try the £7.50 one and see how you get on. Less data and mins but unlimited texts still.
If you decide they're for you, I would appreciate you using this link. I get £5 credit and so do you.
http://giffgaff.com/orders/affiliate/lizfish2Happy shopping, and congratulations on nearing the end of what is probably an eye-watering monthly contract! ;-)
***Mods. I did check the rules and it just says no spam. Which this is not intended to be. Please feel free to remove if I have stepped over the virtual line***