Is there any quality difference using fresh or instant yeast?
Nigella uses the easy blend (also known as instant, quick dried or breadmaker) yeast in most of her bread recipes, such as Hot Cross Buns and Soft White Dinner Rolls (both from FEAST). This type of yeast is dried yeast in fine grains, that can be mixed directly into the flour. Fresh yeast may give a slightly better flavour, but for most home bakers dried forms of yeast are easier to buy and store and these factors outweigh the advantages of fresh yeast. However it is important to use the correct amount of yeast for the recipe and you should use, as often as possible, the type of yeast specified in the recipe.
As a rough guide the ratios for substitution for fresh yeast : dried yeast : instant yeast are 3:1.5:1. So if a recipe asks for 7g instant yeast you should be able to use 10.5g dried yeast and 21g fresh yeast. Dried and fresh yeast need to be dissolved in some warm liquid, with a little sugar, and left to activate before they are added to the flour mixture. Whatever type of yeast you use you should also always check the use by dates on the packaging, as old yeast will not be as active and may lead to a heavy bread.