Place the strawberries in a large heat proof bowl and cover tightly with plastic wrap. Place over a pot of simmering water and allow to cook for 1 ½ hours to extract all the juice from the fruit.
Line a sieve with muslin or cheese cloth and place over a large bowl. Pour the cooked strawberries with all the juice into the cloth and allow to drain for 30-45 minutes. Do not press the fruit as this will make the jelly cloudy. Discard the pulp.
Soak the gelatin leaves in cold water to soften.
Pour the drained juice into a measuring jar and add lemon juice and liqueur. Make up the volume to 600ml with water and transfer to a pan. Place over heat and add the caster sugar, stirring to dissolve. Remove from heat.
Add the gelatin leaves to the warm liquid and stir until completely dissolved.
Pour into moulds and allow to cool for about 15 minutes. Refrigerate for 2 hours or until set.