Photograph: Jonathan Lovekin
Loosely line a loaf tin (or plastic freezer box) approximately 16 x 10cm and 9cm deep with clingfilm or waxed paper.
Put the cranberries in a wide pan, add the sugar and place over a low heat for 5-8 minutes till the sugar has melted. Keep an eye on the pan so the sugar doesn't burn, shaking it from time to time, but avoid stirring as this will crush the fruit. As soon as the sugar has melted and there is a little red syrup in the bottom of the pan, remove from the heat and cool quickly by placing the pan in a sink of shallow cold water.
Pour the cream into a chilled mixing bowl and whisk till soft and thick – stop just before the cream forms stiff peaks.
Crumble the meringues into the cream, but don't stir them in – I like to keep the pieces quite large and chunky. Add the orange zest and vanilla extract, then spoon in the cooled cranberries and their syrup. Very gently fold all the ingredients together. Four or five deep folds with a large spoon is enough – any more than that and you'll lose the contrast of sweet, sour, crisp and soft.
Scrape the mixture into the lined tin, cover with clingfilm and freeze for at least 6 hours. You'll find the parfait easier to cut if you take it out of the freezer 30 minutes before serving and put it in the fridge. Turn it out on to a plate, peel off the clingfilm and serve in thick slices. I like a crumbly finish but if you want something smarter, use a hot, slightly damp knife to slice the parfait.
Get ahead: make up to 1 week ahead.