Pre heat your oven to gas mark 3 and line or grease a loaf tin.In a small bowl, place the ground flaxseed and whisk in 3 tbsp. of water. Set aside.Put flours, xanthan gum, baking powder & soda, salt, spices into a large bowl and mix well.In a separate bowl, mash 3 of the large bananas. Then add the sugar, margarine, vanilla, flaxseed mixture and milk, mash together.Pour this mixture into the flour mixture and fold together until fully incorporated.Pour into your loaf tin.With the remaining banana, slice lengthways and place on top of the batter, pushing it in slightly.Place in the oven for 55 mins to 1 hour. Use a cocktail stick to test if it comes out with a few moist crumbs attached before transferring to a wire rack to cool.