I was just in my local Tesco where they are doing the £6 deal. Not too bad but I used the self-service tills and I was charged the full £10 - no reduction applied. 3 minutes at the customer service desk got me double the difference (though they claimed it didn't count on self-service + 101 other excuses). Overall cost, £2 for a case of 15 San Miguel.
You'll need to be quick before they update their systems though.
BTW, if you are anything like me £25 of that £30 min spend can be used on the 2 70cl bottles of Jack Daniels for £25 offer.