Working in batches, place the apples, plums and onions into the work bowl of a food processor and grind until fine. Transfer the ground fruit and onions into a large pot and stir in the malt vinegar, sugar, salt, Worcestershire sauce, garlic, ginger, nutmeg, allspice and cayenne pepper until the mixture is well combined. Bring to a simmer over low heat and simmer until brown and thickened, about 3 to 4 hours.
Sterilise the jars and lids in boiling water for at least 5 minutes. Pack the sauce into the hot sterilised jars, filling the jars to within 5mm of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat then carefully lower the jars into the pot using a holder. Leave a 5cm space between the jars. Pour in more boiling water if necessary until the water level is at least 3cm above the tops of the jars. Bring the water to a full boil, cover the pot and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface several cm apart until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool dark area.