If it calls for bread flour then they mean strong flour. The US for example doesn't use the word strong, bread flour is the term, and they both mean flour with enriched gluten content. The gluten content on flour varies, you can compare them by looking at protein content, as that is what gluten is, the higher the protein level the more gluten there is.
If you want to weaken strong flour you'd simply add plain flour, just make sure it comes out to the same weight or volume. One thing to keep in mind is the amount of water you add, as if you reduce the gluten content it won't require as much water.
Without seeing your recipe I can't be certain, but it sounds like strong flour is what you want. There is such a thing as extra-strong flour but I've never used it.