The original version of this flapjack recipe is in the Hamlyn All Colour Cook Book, first printed in 1970 ( I bought my copy in 1978). Mary Berry wrote the baking section and I loved her recipes. When I was on GBBO she very kindly signed my copy.
Over the years I have developed the recipe to include lots of different ingredients but M&M's are the all time winner. The addition of cinnamon adds warmth and the vanilla helps the chocolate come through.