We're Giff Gaff too and I buy a Goody Bag when we're away but use their PAYG rates when at home as they're much cheaper when you're not using data and only a few calls and texts. I purchased a £10 credit at the end of September, which had dwindled to £6 something by the time they gave out their Payback which gave me another £4.50 or so and my balance now is still £6.50. So I've spent £8 on calls, texts and data in 3+ months. Not bad.
Of course calls and texts to Mrs SDA who's also on Giff Gaff are free.