To make the jelly, add the strawberries to a saucepan with the 150 ml (1/4 pint) water and the sugar, cover and cook gently for 5 minutes. Sprinkle the gelatine over the remaining 4 tablespoons water in a small bowl and leave to stand for 5 minutes.
Purée the cooked strawberries in a liquidiser or food processor with the raspberries then press the purée through a sieve back into the saucepan. Add the bowl of gelatine to a small saucepan of gently simmering water, so that the water is halfway up sides of the bowl. Heat for 4-5 minutes until the gelatine becomes a clear liquid. Stir into the fruit purée with the lemon juice.
To assemble the trifles, divide the diced Swiss roll between 10 small glasses each about 200 -250 ml (7-8 fl oz) and drizzle each one with 2 teaspoons of sherry. Top with the remaining sliced strawberries then cover with the fruit jelly. Chill for 3-4 hours until the jelly has set, or make the night before if preferred.
A few hours before serving, divide the custard between the trifle glasses. Lightly whip the cream with the vanilla and sugar then spoon over the custard. Return to the fridge with the jar lids until ready to serve. Add decorations, if using at the last minute.