Before you vote cold because they are always this price or Tesco's value potatoes are cheaper, please note the varieties that Somerfield are putting into the bags lately!
Somerields value potatoes are the ones in the big yellow/white plastic bags and you usually you get the not-so-tasty varieties like Saxon or Marfona etc. The last few weeks I have checked the label and found they contain King Edwards & Maris Pipers, which have been good value. But this week they have been filled with Vivaldi potatoes, which IMO are the tastiest around! Have a look at how much Sainsburys charge for them!
This could obviously vary from store to store but it always pays to read the label!