So today I called them up to see what I could do about it, and discovered that my data tarrif is an immensely old one - 10MB a month for £7.50 and that the new one gives you 250MB a month for £7.50. But, of course, they don't switch you over unless you phone up to complain.
Oh, and I was also paying £5 a month for free weekend calls. When I don't use all of my inclusive minutes as it is.
So that should save me about £25 a month.