Halve the mandarins and squeeze the juice. Scrape out the pips and flesh of the mandarins and put them in a pan with about 300 ml water.
Bring to a boil and simmer for half an hour, adding more water if necessary.
Flatten the mandarin skins, put in a pan with 500 ml water, cover and simmer gently for about 2 hours.
Drain the skins, reserving the liquid, and cut them into strips.
Sieve the pip liquid into a heavy pan, add the skin liquid and the sugar and stir over a gentle heat until the sugar has dissolved.
Add the chopped skins, the mandarin juice and lemon juice and cook over a medium heat for about 20 minutes.
Allow to cool for 10 minutes then pour into sterilised jars and seal.