Makes about 6 jars.
Scrub the clementines clean, remove the stalks and use a sharp knife to chop them into thin slices, no more than a couple of mm thick; then cut the slices into small pieces - the size of the pieces of zest will be the size of the pieces of shred in the marmalade so you can slice them really finely if you like small bits, or thicker if you prefer a chunkier marmalade. Reserve the pips then tip the sliced clementines and any juice into a large, heavy bottomed saucepan.
Juice the lemons into the pan, reserving the pips; then place the reserved lemon and clementine pips into a piece of muslin cloth, tie it tightly and place in the pan. Alternatively you can use a tea infuser.
Add the water to the pan then bring to the boil. Turn down to a gentle simmer and cook for 2 hours, until the peel is tender. At this point you can remove the pan from the heat, cover, and set aside overnight before proceeding; or you can proceed straight away.
Remove the bag of pips from the pan and allow to cool until you can safely handle it. Squeeze the bag over the pan making sure that you extract as much juice as possible.
Sterilise about 6 jam jars and lids by washing them in hot soapy water, rinsing well, then placing in a low oven until dry. I just leave them in the oven until needed. Place a couple of saucers in the freezer.
Add the sugar to the pan and place over a gentle heat. Stir until the sugar has completely dissolved then turn up the heat. Cook at a rapid boil until the marmalade reaches 105C/221F, about 10-15 minutes.
To check if the marmalade has reached setting point, remove the pan from the heat and place a teaspoonful on one of the frozen saucers, place in the fridge for a minute then push at it with a fingertip; if the surface wrinkles when pushed then it is ready. If not, bring it back up to a rapid boil for another five minutes then test for a set again.
Once the marmalade has reached setting point, remove from the heat and leave to settle for 15 minutes before ladling it into the warm sterilised jars (never pour hot jam into cold jars). Place the lids on and set aside until cool. Store in a cool, dry place.