Top and tail the gooseberries, put them in a medium-sized pan with the elderflower cordial and bring to the boil. Lower the heat and cook for 5 minutes or so until they are opaque and starting to burst. Remove from the heat, tip into a bowl and set aside to cool. Refrigerate for an hour.
Start making the custard, but first have ready a large mixing bowl with plenty of ice cubes in it. Warm the milk in a medium-sized pan until it starts to bubble and rise up the sides. Remove from the heat. Beat the egg yolks and sugar together for a minute or so until thick and creamy, then add 1 tsp of vanilla extract. Pour the hot milk over the egg and sugar mixture and mix thoroughly.
Wash the milk pan, then pour the custard into it and place over a moderate heat. Stir almost constantly, getting right into the corners of the pan, until you feel it start to thicken. When it is the consistency of double cream, remove from the heat, pour into a clean bowl and place in the ice. Stir the custard for a minute or two then leave to chill. (If at any point it shows signs of curdling, remove from the heat, pour into a cold bowl and beat very hard with a balloon whisk.)
Break the biscuits or sponges into pieces and put a handful in the bottom of each of four dishes. Soak the sponge with the elderflower cordial. Crush the gooseberries with a fork and then spoon them and their juice over the crumbled biscuits.
When the custard is chilled, trickle it over the gooseberries. Cover and refrigerate for a couple of hours. Whip the cream until it can hold a loose shape, then divide it between the dishes. To make the crisp topping, melt the sugar in a pan, add the water, and let it bubble for a minute or two. As it darkens to the colour of toffee, scatter in the crumbled biscuits, stir and tip on to baking parchment. Scatter over the cream and serve.