Place the syrup in a small heavy-based saucepan over high heat and bring to the boil. Whisk in the agar and boil for 10 seconds. Strain the syrup through a chinois into a container and add in the violet syrup and colour. Stir well. Place the container over iced water and refrigerate for 3–4 hours until set.
Break up the gel using a hand-held blender. Pour the gel into a vita-prep (or hi