The Winebank works by debiting you £20 per month, then crediting your account with £20 every 3 months. Once you have signed up and started paying, you can order whenever you want. I got a similar initial offer a couple of years ago, and bought a very cheap case of wine. I used to order fairly regularly from Virgin, but have found lately that prices in the likes of Tesco or Lathwaites are often equally competitive and I don't have to buy a whole case at a time (which is not so good if you discover you're not too keen on a particular wine and you have three or four bottles to get rid of).
Oh, and as others have said, they deluge you with emails and offers forever more.