To make the pie filling: Put the sugar and water into a large saucepan and heat gently, stirring until the sugar has dissolved. Don't let it come to a boil — or if it does, let it cool some before the next steps.
Remove from the heat and sprinkle over the powdered gelatin. Use a whisk to combine and dissolve the gelatin, returning the pan to the heat for a moment if it doesn't dissolve fully. Once totally combined, add the lemon essence and citric acid, and whisk through.
Let the mixture cool, then put the pie case, still in its tin, into the fridge. Carefully pour the liquid filling into the pie case and leave it to set in the fridge, for 5-6 hours, or overnight.