OK, so this is the idea: sauce Marie Rose goes Moroccan. Instead of the usual tomato ketchup (well, be honest) I dollop in some good bought harissa. It works wonderfully, and the honey makes up for the sugar you lack by giving up the ketchup. Be careful when you buy harissa though, as not only does it vary enormously in strength but also some jars that bear the name are not really harissa but some paste made red with beetroot and carrot. Check the back of the tub for ingredients. I used a gorgeously mild harissa here, a rose harissa, that I find poetically desirable. But start with a small amount of the harissa before working up to the amount I have stipulated below. I love prawns/shrimp with their shell still on; a bit of DIY at the table always seems to help the atmosphere. But if you hate mess and can't bear the sight of seeing people dropping shells all over the place and haven't got time for treatment, then simply buy peeled prawns/shrimp instead.