Top and tail and peel the beetroot (you might want to wear clean rubber gloves for this).
Using a mandolin grater or a sharp knife and a steady hand, cut the beetroot horizontally into wafer thin slices.
Pack into the Kilner jar.
Put the remaining ingredients into a saucepan and heat, stirring, until the sugar has dissolved.
Bring to a boil, pour the spiced vinegar over the beetroot and seal the jar.
Once cooled, refrigerate for at least 24 hours before eating. This will keep in the fridge for two weeks.