Butter a 22x33cm shallow baking tray or line with baking paper.
Place the flour, oats, sugar, bicarbonate of soda and salt in food processor. Process for a few seconds. Add the butter and process until it looks like breadcrumbs.
Add whisked egg through feeder tube and process for a few seconds until it starts to stick together.
Use back of a metal spoon to press 3/4 of the mixture into the prepared tray.
Melt the milk chocolate, not the Mars bars, in a bowl placed over simmering water. Pour chocolate over the base and even it out. Leave margin of 2cm around the edges. Put the tin in the freezer for 10 minutes or in the fridge for 30 minutes until the chocolate hardens.
Preheat the oven to 180 degrees C.
While the oven is heating bring the thickened cream to boil in a heavy saucepan then lower heat to simmer. Add Mars Bars and stir until melted; 5-10 minutes. Whisk well until creamy.
Pour immediately over hardened chocolate. Spread out leaving narrow margin.
Crumble remaining oat mix on top so it roughly covers top. Bake for 25 minutes or until edges are golden brown
Loosen edges with a knife and allow it to cool completely until the toffee hardens; 1 1/2 to 2 hours.