Crush the digestive biscuits into fine crumbs using a food processor or high-powered blender. Make sure there are no lumps in there.
Melt the butter and add it to a bowl with the biscuit crumbs. Mix well until the crumbs are evenly coated in melted butter. Press this mixture into the base of an 8-inch springform tin.
To a large bowl, add the cream cheese, icing sugar, raspberry extract, lemon juice and blue food colouring. Mix well.
Add the double cream and use an electric whisk to whip it up until thick. Spread the filling evenly over the biscuit base.
Refrigerate the cheesecake overnight to set, or for at least 8 hours.
Gently remove the cheesecake from the tin, using a butter knife or palette knife to loosen the base. Carefully transfer it to a wire rack or plate.
Top with fresh raspberries and enjoy!