Put your two leaves of gelatine in a bowl of cold water to soak.
Put the frozen berries, caster and lemon juice into a pan on medium heat, once it has started to bubble away reduce the heat to low and let it simmer for around 10-15 minutes, be careful it doesn’t catch on the bottom and burn, give it an occasional stir.
After around 10-15 minutes it should have thickened a bit, take it off the heat, squeeze the excess water out of the gelatine and add the leaves to the raspberries, whisking them in.
Allow the mixture to cool a bit before you add your gin as you don’t want the alcohol to evaporate, add a few drops of rosewater, to your taste. Note you aren’t trying to make it taste of rose just bring out the botanicals in the gin and lift the flavour a bit, so have a taste, see what you think and add a little more if you think it’s needed.
When you are happy, place the raspberry mixture into a jug are carefully pour on top of the sponge fingers, try to make them all even, if your sponge fingers float up to the surface just use a spoon to push them back under the raspberry jelly. Leave them to set in the fridge for a good few hours until you feel the jelly layer is set.