It states in your books that you use large organic eggs, which I presume are hens eggs, my question is would it be ok to use organic duck eggs please?
Nigella does use hens' eggs in her books as they are the most commonly available. However it is possible to use duck eggs instead. Duck eggs tend to be slightly larger than hens' eggs but they also have thicker shells so the difference in the liquid volume of the egg is quite small and you should be able to substitute the eggs on a one-for-one basis for large hen eggs.
Duck eggs have a slightly richer yolk so you may notice this in items like French toast and omelettes. They also have slightly more protein in their whites so the eggs are wonderful for baking. Generally cakes made with ducks' eggs will rise slightly more and be slightly richer, and they may also last a day or two longer. The thicker shell also means that duck eggs will store for longer than hen eggs (up to 6 weeks in a cool environment).