Sieve the flour and salt together in a large mixing bowl. Make a well in the centre of the flour and break the eggs into it. Whisk the eggs using a fork mixing the flour into the eggs as you whisk. Gradually the milk and water mixture, still whisking (don't worry about any lumps as they will eventually disappear as you whisk). When all the milk and water has been added, use a spatula to scrape any leftover bits of flour from around the edge of the bowl and finally whisk once more until you get a nice smooth batter.
Melt half butter in a pan. Spoon 2 tbsp of it into the batter and whisk it in, then use the rest to grease the pan. If you think the pan is too dry just add a smidge more butter. Make sure the pan is really hot to start with and then turn the pan down to medium. Use a ladle to drop the batter into the pan and swirl it around the pan from side to side. It will only take a minute or so for one side to become golden pan. Use a fish slice or if you’re feeling brave flip the pancake over! The other side will only need half a minute so. Once cooked, slide it onto a plate and keep warm until ready to serve. Fold each pancake in half and then into quarters. Add the caster sugar into a non-stick frying pan and heat over a low-medium heat. Allow the sugar to melt gradually without stirring. Continue cooking until it becomes a bright caramel colour. Take the pan off the heat and add the orange zest, lemon juice, grand Marnier and then return to a low heat to re-melt the caramel into the liquid. Add the butter to the sauce and bring to the boil simmering gently until the sauce reduces. It should be slightly thick and glossy. Add the pancakes to the pan and warm through. Serve with cream or ice-cream.