Store in an airtight container in a cool place for 5 to 7 days. In hot weather (or if the central heating's on) keep in fridge. The cake can be made ahead and frozen for up to 3 months (though the nuts may soften slightly on defrosting). Wrap the fully cooled cake (still on the springform tin base) tightly in a double layer of plastic wrap and a layer of foil. To defrost, unwrap and leave it (still on the tin base) on a plate at room temperature for about 4 hours.