Blood orange marmalade

Method

1. Wash all the oranges, scrubbing to remove any wax. Halve, then squeeze the juice through a fine sieve into the pan. Cut the skins into quarters, then slice the peel as thinly as you can, adding it to the pan as you go. Take any pips and pith from the sieve, put in the middle of the muslin and tie with string. Add the lemon juice and 2 litres cold water to the pan, then tuck in the bag of pith/pips. Simmer for 1½ hours – the peel will be soft. Squeeze out the bag, then discard.
2. Add the sugar to the pan, stirring until dissolved. Turn up the heat and bring the pan to a rolling boil, then simmer briskly until the thermometer reads 104.5°C (it could take 15-30 minutes). Take the pan off the heat immediately and leave for 15 minutes.
3. Ladle the warm marmalade into sterilised jars, then seal with the lids.