Mix the butter and biscuit crumbs together. Tip into a 20cm (8 inch) loose-based cake tin or deep flan dish and press in an even layer over the base. Chill for 20 minutes.
Pour the boiling water into a measuring jug. Stir in the powdered gelatine. Leave to dissolve for 10 minutes, stirring occasionally.
Meanwhile, beat together the cream cheese, sugar, lemon zest, lemon juice and yogurt. Purée half the strawberries in a blender, then fold this purée through the cream cheese mixture with half the blueberries.
Pour the cooled gelatine liquid into the mixture in a thin, steady stream, stirring thoroughly. Pour over the biscuit base. Chill until set, about 2-3 hours.
Remove the cheesecake from the tin and arrange the remaining strawberries, blueberries and raspberries on top.
Freeze undecorated, either whole or in portions, for up to 3 months. Defrost overnight in the fridge. Purée some extra Spanish strawberries, sweeten with a little caster sugar and serve as a sauce with the cheesecake.