Preheat oven to 180 C / Gas 4. Grease and flour a 23cm x 32cm rectangular cake tin.
Mix brown sugar, chopped walnuts and coconut together for the topping. Set aside.
In a large bowl, mix together the flour, sugar, bicarbonate of soda and salt. Make a well in the centre and pour in the eggs and tin of fruit cocktail with juice. Mix well and pour into prepared cake tin. Sprinkle with topping mixture.
Bake in the preheated oven for 30 to 35 minutes, or until a skewer inserted into the centre of the cake comes out clean.
In a saucepan, combine margarine, golden syrup, evaporated milk and desiccated coconut. Cook, stirring constantly, until mixture boils and thickens. Pour over cake and place back in oven for few minutes until it bubbles. Cool before serving.