To be clear - this deal is for anyone who wants to use their own sim in a Galaxy S3 and who hasn't got the money to splash out 500 sheets on a sim free version.
It's a familiar theme - £27 per month which you can reduce to £21 per month by removing the 500mb internet bolt on at outset. The phone cost is £49 which is partly offset by £35 Quidco/TCB.
So overall cost is £519 assuming cashback tracks and the icing on the cake for me anyway is that you get free insurance over the contract. I've no idea how valuable Tesco Mobile Phone Insurance so would be interested in people's comments on this.
Hope this helps someone out.