As for tins of tomatoes there are some surprises concerning them too. I thought one tin of toms was pretty much identical to another. However, my daughter went on the Atkins no-carb diet and took to reading labels to identify items with hidden sugar, she told me that, for example, Tesco's own brand tomatoes contained much more sugar than brands like Napolina so we both pay the premium and buy the Italian brands now.
Also, I read somewhere that Waitrose and M & S are much better on the 'no added salt and sugar' front than other shops.
I have never bought ready made pasta sauces but I do buy pesto and had no idea it contained sugar.
Makes you think!