Wash, stone and quarter the plums. Place in a deep, heavy bottomed non reactive saucepan with the water. Bring up to the boil and simmer for about 15 minutes until the plums are soft and have started to break down.
While the plums are cooking sterilise your jars, heat the oven to 140C/275F/gas mark 1. Wash the jars and lids well in hot soapy water then rinse. Place a sheet of baking parchment on the shelf of your oven and place the jars and lids on top of it, making sure that they aren't touching (don't dry them), leave them in the oven until you need them (always sterilise more jars than you think you will need). Place three saucers or small plates in the freezer.
Add the lemon juice and sugar to the plums and heat gently while stirring until the sugar has dissolved. Bring it up to a fast boil and boil for 10 minutes then remove from the heat and check for a set by putting a small blob of the jam on one of the frozen plates, put the plate in the fridge for a minute until the jam has cooled then push at it with a finger, if the jam wrinkles when you prod it, it is ready.
If the jam isn't ready yet, bring it back up to the boil and boil rapidly for another five minutes, then remove from the heat and test for a set again. Repeat until the jam is set. Alternatively, if you have a thermometer, the jam should reach 105C/220F (setting point).
Let the jam cool for about 10 minutes, then stir in the amaretto. Divide the jam between the hot jars (never add hot jam to cold jars, they may shatter) and place the lids on. Store in a cool, dark place and refrigerate after opening.