Grease and lightly flour a 26cm ring tin. Sprinkle about 100g of the chocolate chips along the bottom of the tin.
Beat together the margarine and caster sugar using the cake beater attachment on an electric mixer. Scrape down the sides frequently with a non metal spatula, until light, pale and fluffy.
Beat in vanilla extract. Beat in eggs one at a time, beating well after each addition. If mix looks like it is curdling, beat in 1 tablespoon of the flour. Scrape down the sides after each egg has been beaten into the mix.
Mix baking powder with flour. Add half to butter mixture, mixing on slow speed until well mixed. Add other half of flour and repeat.
Take bowl off the mixer stand. Using a non-metallic spatula, mix in 2 tablespoons of orange juice and the orange zest along with the rest of the chocolate chips. Spoon mixture evenly around the ring tin, level and smooth the top, and tap tin gently on counter top to remove air bubbles.
Place on oven shelf in middle of oven and bake 35 to 40 minutes if using a fan oven, or a little longer if using regular. A skewer inserted in the centre should come out clean and the cake should just start to shrink away from the sides.
Whilst cake is baking, place about 4 tablespoons of orange juice in a small pan, add icing sugar to taste and heat gently until sugar is dissolved, then boil rapidly until slightly thickened, stirring constantly.
Remove the cake from the oven and place on a wire rack. While still hot, poke lots of holes into the cake and spoon glaze over, then leave to cool in tin. When almost cool place a piece of greaseproof paper over the tin and gently turn over; paper should be on the bottom now. Cake should slide out. Remove tin and leave to cool completely.