Kiwifruta: Could someone tell me what difference a little flour makes when activating the yeast? I haven't come across a recipe that does this before, apart making poolishes etc.
Maybe this Wikipedia article helps explain:
A pre-ferment and a longer fermentation in the bread-making process have several benefits: there is more time for yeast, enzyme and, if sourdough, bacterial actions on the starch and proteins in the dough; this in turn improves the keeping time of the baked bread, and it creates greater complexities of flavor. Though pre-ferments have declined in popularity as direct additions of yeast in bread recipes have streamlined the process on a commercial level, pre-ferments of various forms are widely used in artisanal bread recipes and formulas.
I'm going to hazard a guess that page on wiki is incomplete. As yeast produces the enzyme amylase, which breaks down starch in the flour, so apart from sourdough pre-ferment, using it in a standard pre-ferment probably gives the dough a head-start in amylase level in the dough.