Put the cornflour, mustard powder, mustard seeds, cumin, coriander, turmeric, ginger and 120ml of the vinegar in a bowl and stir to a runny paste. Put the rest of the vinegar and sugar in a pan and heat gently until the sugar has dissolved. Pour the hot vinegar over the paste, whisking until loosened, then pour everything back into the pan and whisk until it comes to a simmer and starts to thicken. Simmer for 2-3 minutes then pour over the veg and mix well. Spoon into sterilised jars, seal and leave in a cool, dark place for at least 3-4 weeks for the flavours to develop. Once opened, keep in the fridge and consume within 2-3 weeks.