Preheat the oven to gas 4 / 180C / 350°F and line an 8"x 8" square cake tin with parchment paper
Combine the margarine, oats, flour, brown sugar, bicarb of soda, xanthan gum if using and salt in a large bowl and knead together. If using plain flour only add 200g of the butter
Press half of the oat mixture into the bottom of the lined tin until well packed down and set the other half aside. It can be a bit hard to spread the mixture to meet the sides so I put my hand inside a small ziplock bag and it helps pat it out without it all sticking to you
Put the chopped dates and raisins in a saucepan with the white sugar and water
Bring to the boil then reduce to a simmer for 5 - 10 minutes or until the water has mostly evaporated leaving a date paste in the pan
Spread the paste over the packed oat mixture
Press the rest of the oat mixture that was set aside on top of the date paste. It's easier if you break pieces of dough off and pat them flat between your palms and put them on top like a patchwork quilt. Then pat the whole surface down when you're finished
Bake for 30 minutes then let cool completely before slicing into squares
Delicious!
