Got ours for £4 with topcashbackIt was a cashback deal from affordablemobiles. You have to email them copies of your bills at regular intervals and they credit you back. You pay the full amount (£18) a month, but when you get the cash back that brings the "effective price" to £6 a month. The deals come and go regularly, if it's not there today it might be later on.
Just checked. It's still there, under SIM only three.