Add the vinegar, sugar, garlic and ginger to a preserving pan, then add the dried chillies, allspice, salt and peppercorns. Heat gently, stirring from time to time, until the sugar has dissolved, then simmer gently for 10 minutes so that the flavours mingle together.
Stir in the pineapple and sliced mango and cook over a medium heat for 10 minutes until the mango is just translucent and the liquid is syrupy.
Ladle into warm, dry jars, filling to the very top. Press the mango slices beneath the syrup, then cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.
To serve, partner with poppadums.