Gluten free Parkin for Bonfire Night

Parkin is a firm British favourite for Bonfire Night. For many Northerners the Bonfire Night is almost unthinkable without a big slice of this dense spicy cake.
As we have invited friends over for Bonfire Night meal, with hot drinks and all the roasting marshmallows' caboodle, I fancied baking a spicy parkin. One of our guests is on a gluten free diet, so off I went googling for the right recipe.
One that caught my eye is a recipe found on All Recipes - see Gluten Free Parkin. I followed the recipe and method closely enough, but adapted it to what we have in the kitchen. I bought a fresh pack of gluten free flour. I have also used only one type of gluten free flour as opposed to three used in the recipe above, I didn't want to buy 3 different products, which I don't really need.

Gluten free parkin
Ingredients:
110g margarine (I used Stork)
100g golden syrup
70g black treacle
225g gluten free flour
1/2 tsp xanthan gum
1tsp baking powder (gluten free)
110ml milk
2tsp Waitrose signature spice (cinnamon, ginger, nutmeg, cardamom, allspice, star anise, black pepper, tangerine oil, cloves)
1tsp ground ginger
2 medium eggs

Melt margarine with golden syrup and black treacle in a saucepan. Set aside, and add milk, stir together and allow to cool.
Preheat the oven to 180C, and line a square cake tin (brownie tin) with parchment paper.
In a deep mixing bowl sift in the flour, xanthan gum and baking powder. Mix in the spices, and beat in the eggs. Add the milk/margarine/syrup mix, and combine together well to the consistency of thick yogurt. The addition of xanthan gum adds the viscosity to the batter.
Pour the batter in the cake tin and smooth the edges to even it out.
Bake for 50+ minutes. Check readiness with a wooden toothpick.
Cut into squares, once it's cold.

I forgot to add sugar, but the cake was sweet enough, thanks to golden syrup and treacle.
If you are not on a gluten free diet, obviously use the standard flour, which I think will improve the flavour. I am not the biggest fan of gluten free flour, it just doesn't taste the same, but we are lucky that we don't need to use it often.
Maybe the combination of three different flours would be better, as in the original recipe. I used the Doves Farm gluten free flour which is a combination of rice, potato, tapioca, maize and buckwheat.

Let's say it was not my best bake. You might ask why I blog about it, if it's not perfect. It is more of a diary and a cooking reference, so that I would know what I have baked, how, and what to do to try to improve it in the future. It needs something to make it less dry, perhaps swap margarine for oil?

If you have suggestions, please let me know, how you make your gluten free bakes moist.