willow would grow in the gaps , some hedgers frown on it being used though . Sheep and goats will eat it though , some places I have heard of it being grown specifically as goat fodder. Ash will also grow in the gaps , as will any of the hedging types, but willow will fill the gaps a bit faster maybe .You could try just taking some cuttings from your other hedge plants and poke them in , if they don't take it won't have cost anything ? If the gaps aren't too big , you could pull some of the nearest branches across and weave them in the branches of the bush the other side of the gap, or go mad and lay the whole hedge......It all depends on what you want to do really.
cheers
Russ