The first step is to give the lemons a really good scrub, and remove the eyes (the little circular bits that come off the ends, where they used to join on the tree).
Then they need to be cut in half and juiced. Save the juice.
Remove the pips, and save them on a saucer.
Now slice the skins of the lemons into the thinnest slices that you can possibly manage without losing any of your digits.
Place the slices into a large non-metallic bowl, add the lemon juice and give everything a bit of a stir.
Place all of the saved pips onto a piece of muslin (you can buy this in any good kitchen shop), and tie to make a little package with a piece of string. The idea is to hold the pips and not let them mix through the marmalade. They will help your mixture to set, and the parcelling-them-up business will make it easier to discard them before bottling.
Place the muslin parcel of pips in the bowl with the sliced lemons in their juice and add 2.8 litres of fresh water. Give the mixture a bit of a stir-about, and leave covered in a clean t-towel overnight so that the acidity of the liquid will soften the skins.
Day 2 - roughly 24 hours later.
Take an old saucer and put it in the freezer to get it crazy cold. You will use this to test the consistency of your marmalade later in the process.
Put your lemon skin/juice/pips/water mixture in a large saucepan and bring to the boil. Leave to boil gently for about two and a half hours until the skins are soft and slightly translucent.
While you’re waiting for the skins to cook you should sterilise your jam jars. Wash both the jars and their lids in hot soapy water. Rinse well with warm clean water. Dry the outsides but not the insides. Place them on a baking tray (open top ends up) with the lids (top sides up) on a separate tray in an oven pre-heated to 150 degrees Celsius for about 20 minutes to half an hour.
You should also ‘warm’ your sugar. With about half an hour to go before the lemons are ready, weigh it out and put it in the oven at about 150 degrees Celsius in a large flat dish.
After the lemons in the saucepan have been boiled to the point where they are soft and translucent, add the sugar, citric acid and the lavender seeds with a knob of butter (this helps to stop it burning, according to my mum) and bring to a rolling boil, which is a gentle boil that isn’t too violent – think of gently churning rapids downstream from a large waterfall. Stir occasionally and let everything boil for another fifteen to twenty minutes or so until it starts to set. You should use your thermometer for this stage to check the temperature. The marmalade will start to set at around 106 degrees Celsius.
When it reaches this stage, and you see it starting to thicken, you need to do the frozen saucer test.
Take your saucer from the freezer and drip a little drop of the marmalade onto it. Leave it for a few seconds and then gently push it with your fingertip. If the marmalade is ready it should have formed a thin skin on top and you should be getting wrinkles when you apply pressure. It may still seem to be too liquid for spreading on your toast, but if there is a very thin skin with wrinkles you are good to go. The mixture will thicken/ solidify a bit more once it cools. This is the trickiest stage of the process, so do keep your eyes peeled.
Anyway if you haven’t reached this point let the mixture boil on for another three or four minutes and test again. Keep going until you get that skin-and-wrinkle-thing happening on your saucer, but be really careful as it can easily burn if it overheats (and, believe me, cleaning the bottom of the saucepan when it does is a nightmare job).
When the magic point arrives (skin and wrinkles on the saucer) take the saucepan straight off the heat, and let everything sit for about 10 minutes before starting to bottle it into the jars. This slight cooling should result in the optimum amount of peel being dispersed through the mix rather than all settling at the bottom of the jars.
Remove your little parcel of pips and fill each jar to the neck. I use my oven gloves and a soup ladle that I've washed down with boiling water for this part of the operation. Then place a disk of waxed paper on top, and put on the lids.