Wash the fruit, then tip into a preserving pan with the lemon juice and 300ml/1⁄2 pint water. Bring slowly to the boil, and simmer for 30-40 minutes until the fruit is soft.
Carefully pour the contents of the pan into a scalded jelly bag with a large bowl set underneath to catch the juice (see the Step-by-step photo). Leave for several hours.
Measure the juice back into the pan, then add 500g of sugar to every 500ml of juice or 1lb sugar for every pint of juice. Stir over a low heat until the sugar has dissolved, then raise the heat and rapidly boil until setting point is reached. Test this by spooning a little on to a chilled saucer. Cool slightly then push with your finger – if it wrinkles it is ready. If not return to the heat, boil for 5 more minutes and test again.
Pot into warm sterilised jars and cool before sealing. Can be eaten straight away, but keeps for up to a year.
Jars must be sterilised before filling. Preheat the oven to fan 100C/conventional 120c/gas ½. Wash the jars in hot soapy water, rinse well and place on their sides in the oven for 15 minutes.
Britain's most famous damson growing area is the Lyth Valley in Cumbria, says Henrietta Green. There you can buy locally-made damson gin, damson vinegar, all manner of damson chutneys, jams and pickles and even damson cheese.