Shake the elderflower heads to remove any insects. (Rinsing them will remove the pollen and the flavour). Place them in a saucepan with the cream. Bring to a very gentle simmer, remove from the heat and cool. Cover and place in the fridge overnight.
Strain the cream through a sieve lined with a muslin. You need 130ml for this recipe.
Put the plain flour and butter in a bowl and rub together with your fingertips until it resembles breadcrumbs.
Mix in the icing sugar and a pinch of fine sea salt followed by the egg yolk and 1 tablespoon of ice cold water. Mix until you have a combined dough, don't overwork it. Shape the dough into a ball and then flatten it out into a disc. Place in a freezer bag, then chill for 30 mins before using.
Roll out the pastry and line the pastry case. Prick the base with a fork and chill for 10-15 minutes. Preheat the oven 175°C fan.
Line the pastry case with parchment and baking beans and bake for 12 minutes, remove the beans and bake for a further 10-15 minutes or until golden. If a few cracks have appeared, brush with a little beaten egg yolk as soon as it comes out of the oven. Cool for 5-10 minutes before filling.
Reduce the oven temperature to 150°C fan.
Crack the eggs into a medium bowl and add the sugar. Follow with the elderflower cream, lemon zest and juice. Whisk with a balloon whisk.
Carefully pour the mixture into the pastry case, hold back the last few millilitres. Add this when the tart is in the oven, to save spillages.
Bake for 30-32 minutes or until when you give the tart a gentle shake it still has a little wobble in the centre. Cool. A few cracks may appear but you can cover these with berries.
Take the berries and slice in a random fashion, add a tablespoon of elderflower cordial and leave for 20 minutes to macerate. Now decorate the tart and add a light dusting of icing sugar. Best served at room temperature.