Key lime and blackberry cheesecake

SERVES 12

READY IN 30 MINUTES PLUS COOLING AND CHILLING

35g low-fat spread 8 reduced-fat digestive biscuits 250g extra-light soft cheese 500g fat-free natural fromage frais 3½ level tbsp sweetener granules finely grated zest of 2 limes, plus 125ml lime juice (about 4 limes) 3 small leaf gelatine sheets 50g blackberries, thawed if frozen

● Lightly grease a shallow, round 20cm loose-bottomed cake tin with 5g spread and line with nonstick baking paper. Put the biscuits in a sealable food bag, close tightly and bash with a rolling pin to make fine crumbs. Melt the rest of the spread in a pan and stir in the crushed biscuits. Press the mixture into the prepared tin to make a thin base using the back of a spoon.

● Put the soft cheese, fromage frais, 3 level tbsp sweetener and the lime zest in a bowl and beat until smooth. Set aside. Drop the gelatine sheets into a bowl of cold water and leave to soak for 5 minutes.

● Meanwhile, put the lime juice in a small saucepan over a low heat and warm through gently. Scoop the gelatine out of the water, squeeze out the excess water and drop the gelatine into the lime juice. Stir until completely dissolved, then set the mixture aside until it is cool and showing signs of setting.

● Put the blackberries and remaining sweetener into a mini food processor and blend until smooth. Press through a sieve into a small bowl, discarding the bits in the sieve, and set aside.

● Beat the lime juice mixture into the cheese mixture and pour it over the biscuit crumb layer in the tin. Drizzle the blackberry purée over the top and marble it with the tip of a skewer as beautifully as you can. Chill for at least 4 hours or until set.

● Remove the cheesecake from the tin and discard the baking paper. Slide it on to a serving plate and slice to serve.