1: Preheat the oven to fan 180°C/gas 4. Spray a 900g (2lb) non-stick loaf tin with a few sprays of oil.
2: Using a stand mixer or electric hand whisk and large bowl, cream together the margarine, caster sugar and sweetener until light and fluffy. Add the eggs, one at a time, beating well after each addition.
3: Sift the flour, bicarbonate of soda and ground cardamom together over the mixture, add the vanilla seeds and lime zest and fold in gently, using a spatula, until just combined. Lastly, fold in the grated courgettes.
4: Spoon the cake mixture into the prepared tin and gently level the surface. Bake on the middle shelf of the oven for 50–60 minutes. To test, insert a skewer into the middle of the cake: it should come out clean; if not give it a little longer.
5: Once cooked, leave the cake to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
6: To make the icing, in a bowl, whisk the icing sugar, cream cheese and lime juice together until smoothly combined.
7: Spread the icing on top of the cake and sprinkle with the lime zest. Cut into 8 thick slices to serve.