O2 Simplicity 12 months: 600 xnet mins / 300 O2 to O2 mins / unlimited texts / monthly cost £15.50 with £5 discount for six months + £25 shopping voucher + £70 Topcashback (works out £5.08pm after discounts and casback - data can be added)
Looks like a decent deal for a sim only. Was posted some time ago but they appear to have added the £5 for six months discount and increased the O2 to O2 minutes since then. Plus the cashback is currently up to £70 through TCB.
Data costs are:
100Mb = £3
500Mb = £6
1Gb = £10