1. Preheat oven to 175°C/350°F/Gas 4. Grease and line a 900g/2lb loaf tin.
2. Combine all the crumble Ingredients in a bowl and rub in the butter with fingertips until mixture resembles breadcrumbs. Set aside.
3. Sieve the flour and bread soda into a bowl, stir in the candied peel and make a well in the centre.
4. Put the margarine in a saucepan over a low heat and melt. Remove from the heat and mix in the sugar, yogurt, eggs, lemon juice and vanilla essence.
5. Pour into the well and mix well with a fork until just blended.
6. Spoon half the mixture into the prepared tin, smoothing the top. Sprinkle over half the crumble. Put remaining mixture on top, dropping it on in spoonfuls. Spread as evenly as possible. Sprinkle on the remaining crumble.
7. Bake for about 1 hour or until firm to the touch. Leave in tin for about 30 minutes then cool completely on a wire tray.