Put the double cream, sugar, elderflower, lemon juice and nutmeg into a bowl and carefully whisk until fairly firm (this won't take too long as the lemon juice thickens up the cream). Distribute spoonfuls of the remaining gooseberry mixture over the cream and scatter the crushed meringue on top.. Leave in the fridge until the custard has set.To serve, spoon the cream mixture on top of the trifle. Put it in the fridge for an hour or so to set.Meanwhile if you're not using good quality ready-made (not the yellowy cornflour type), make the custard.
Split the vanilla pod in half lengthways and scrape out the seeds with the point of a knife. Put the single cream, vanilla pod and seeds into a small saucepan and bring to the boil. Remove from the heat and leave to infuse for about 10 minutes In a bowl, mix the egg yolks, sugar and cornflour together. Take out the vanilla pod and pour the cream on to the egg mixture and mix well with a whisk.
Return to the pan and cook gently over a low heat for a few minutes, stirring constantly with a wooden spoon until the custard thickens Make sure it doesn't boil. Remove the custard from the heat and give it a final mix with a whisk. Transfer to a bowl, lay a sheet of clingfilm over the surface of the custard to prevent it forming a skin and leave to cool for about 30 minutes.Once the jelly has set, spoon over the custard and leave to set for half an hour or so Now prepare the topping. Then pour over the cooled, but not set, jelly so it just covers the sponge and gooseberry.