Decorate the glasses by pouring 1 tbsp of the liqueur onto a small plate. Dip the rim of the glasses into the liqueur and then into some caster sugar. Allow to dry.
Put 1½ punnets of raspberries into a bowl. Pour over the liqueur and icing sugar and stir gently, until the icing sugar has melted. Leave to stand whilst making the mousse.
Place the gelatine in a bowl of cold water to soften, making sure that it is completely submerged.
Gently pulse the rest of the raspberries in a food processor until they are pulpy, but not completely puréed. Pour into a saucepan with the caster sugar and heat gently until the sugar has melted.
Whilst the raspberries are still hot, lightly squeeze the water out of the gelatine and stir into the raspberries. Transfer the mixture into a bowl and leave to cool (you can place over a bowl of iced water to speed things up).
When the raspberry mixture is cold and starting to set, whisk the lactose free cream to soft peak consistency and gently stir it into the raspberry mixture.
Next assemble the mousse by putting half of the liqueur raspberries in the bottom of the glasses, top with half of the mousse, repeat and chill until set. Decorate with fresh raspberries and a sprig of mint.