Just rang up T-Mobile (0800 956 2208) and mentioned Tescos current sim only deal and they offered me this.
Some people are managing to get cheap deals that includes a phone (slightly different contract)...so its worth asking if you're looking for one.
They said it was only likely to be around for the next week.
It was very easy, i didn't even have to try and push them for a lower price.
Phone number: 0800 956 2208
Dialing voicemail is 10p extra per call