Put the mandarins (skin on) and lemon juice into a large pot big enough to hold them all and barely cover with water.
Bring to the boil, then reduce the heat to a gentle simmer and cook until the mandarins are completely soft and just starting to break apart.
Remove the mandarins from the pot (retain the liquid) and set about removing the seeds (if any).
While they are still hot, do this by tearing them apart with 2 pair of small tongs and just lifting the seeds out – not a completely painful exercise but it does take a little patience.
Add the puree back to the water and take a complete measurement of the fruit and water mixture combined. You are now going to add exactly the same amount of sugar to this mixture – so if you have 1 litre of fruit/ water, then add 1kg of sugar.
Place the pot back on a medium-high heat and bring to the boil, stirring all the time to dissolve the sugar. Keeping on a steady, constant boil, cook for about 20 minutes then do the setting point test. Use the ‘flake test’ (see notes). Once setting point is reached, remove the marmalade from the heat and leave to cool for 10 minutes.
Pour into hot sterilised jars and seal straight away.
The flake test - When you are close to setting point, take a tablespoon of the jam or marmalade with a metal spoon and leave it to cool a little. Now tilt the spoon on its side and pour the mixture onto a plate. If the marmalade drips in a solid sheet, it is ready. If it falls in drips then you are not quite there yet.