Butter a 22x33cm (9x13 in) swiss roll tin, or line with parchment.
Place the flour, oats, sugar, bicarbonate of soda and the salt in food processor. Process for a few seconds. Add the butter and process till it looks like breadcrumbs.
Add beaten egg through feeder tube and process for a few seconds till it starts to stick together.
Use back of a metal spoon, press 3/4 of the mixture into the prepared tin.
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 1/2 inch around the edges. Put the tin in the freezer for 10 minutes, or in the fridge for 30 minutes, till the chocolate hardens.
Preheat the oven to 180 C / Gas 4.
While the oven is heating up bring the double cream to boil in a heavy saucepan, then lower heat to simmer. Add chopped Mars bars and stir till melted (5-10 minutes). Whisk well till creamy.
Pour immediately over hardened chocolate. Spread out leaving narrow margin.
Crumble remaining oat mix on top so roughly covers top. Place on a baking tray. Bake for 25 minutes or until edges are golden brown
Loosen edges with a knife, allow it to cool completely till the toffee hardens (1 1/2 to 2 hours).
Cut into small squares and enjoy!
Make sure you grease the baking tray well and the layer of milk chocolate can be omitted but it tastes great with the layer of milk chocolate. Enjoy!!!!!!!!!