Sterilise five 500ml pickling jars with lids and rings. Keep hot.
Place beetroots into a large pot and fill with enough water to cover. Bring to the boil and cook until beetroots are tender enough to pierce with a fork, but not mushy. Drain and cool, reserving 600ml water from the beetroots. When beetroots are cool enough to handle, peel and trim the ends. Cut into wedges or chunks if desired and pack into hot jars.
Whilst the beetroots are cooking, combine the sugar, vinegar, pickling salt, lemon juice and clove oil in a large saucepan. Bring to the boil. Pour the hot brine over the beetroots in the jars to within 5mm of the top. Wipe the rims with a clean towel and seal with lids and rings. Set at room temperature for 24 hours, then store in the refrigerator. Wait a few days before eating to allow beetroots to soak up the flavour.
Clove oil is also known as oil of clove. Clove oil and pickling salt can be purchased online.