These are supposed to help by encouraging 2 types of root - the deeper, water-seeking roots go down into the bag, while the shallower feeding roots develop in the pot. I've sited the bag against the south facing fence so they should (British summer willing!) get plenty of sun.
Finally, a bit of companion planting - a pot marigold (Calendula). I've also planted some basil seeds today (free with Innocent Smoothies), which will be planted out in front of the tomatoes when they're big enough.
In the meantime I've put out the pot of basil that has been sitting on the windowsill since I bought it (the best before date is 02 Nov 2011!!).