I would like to try the recipe for Nigella's Breakfast Bars (from Nigella Express) but can you tell me if the condensed milk to use is the sweetened one or unsweetened? Thanks.
Condensed milk, as used in the Breakfast Bar recipe, is always sweetened. There is another type of canned milk which is not sweetened but this is known as evaporated milk (though in theory evaporated milk is unsweetened condensed milk).
Both evaporated milk and condensed milk have been cooked so that around 60% of their original water content has been removed by evaporation. Evaporated milk is canned after this process and will have a pouring consistency similar to milk. However to make condensed milk a large amount of sugar is then added to the evaporated milk before it is canned (typically condensed milk is around 40% sugar), making the condensed milk much sweeter and also much thicker.
You should not switch between evaporated milk and condensed milk in recipes. Condensed milk is used in baking, candies and desserts as it is sweet. Evaporated milk may be used in both sweet and savoury dishes.