Heat the oven to 180C/fan 160C/gas 4 and line two baking trays with baking paper.
Mix together the oats, coconut, raisins, candied peel, flour, brown sugar, cinnamon, mixed spice and salt in a large bowl. Put the butter and golden syrup in a small pan and cook over medium heat until the butter has melted. Remove from the heat and add the bicarb and 3 tbsp of boiling water, mixing together until it foams. Immediately pour this over the oat mixture and use a spatula to mix together until uniformly combined.
Divide the cookie dough into 18 pieces and roll into balls. Put onto the prepared baking trays 5cm apart and bake for 15 minutes until golden brown. Halfway through baking, remove the trays and use a glass to press the cookies into a flat disc about 1cm thick then continue baking. Cool for a couple of minutes before transferring to a wire rack to cool completely.
To decorate, put the icing sugar into a large bowl, add the milk and vanilla, and mix together to form a smooth, thick paste, similar to the texture of toothpaste. Scrape the mixture into a piping bag and snip off the end. Pipe a cross onto each cookie and sprinkle the coconut to cover the piped white lines. The cross mixture can set quite quickly so it’s best to work on one cookie at a time.
The cookies will keep for 3-4 days. On the first day the cookies will have a firmer, crisper texture and then they will soften, both of which are equally as delicious.