Seamus, I think Tesco's success largely lies on convenience rather than value or positive shopping experience. It seems there is a Tesco store local to just about everyone. We have 2 - an absolutely enormous store about 15 minutes drive and a small Tesco Express about 10 minutes walk from my house. There are also another 3 stores within driving distance that I can think of - whereas Asda, Morrisons, Sainsburys, Aldi and Iceland all have just 1 (or 2 in the case of Morrisons) store local to me.
The reason they're so successful, is because they're probably about 70% of people's local supermarket.