Place the peppers directly onto a naked gas flame or under a very hot grill and let them blacken and blister. Turn now and then with a pair of metal kitchen tongs. When blackened all over, pop them into a bowl and cover with clingfilm. Leave the peppers to steam for about 10 minutes, then remove the clingfilm and leave them to cool. When they are cool enough to handle, rub off the blackened skin and brush away the seeds from the inside. Tear the mozzarella balls into chunks and divide between 4 plates. Tear the peppers into strips and divide over the plates of mozzarella. Halve the tomatoes and scatter over the top. Season well with sea salt and black pepper and drizzle with olive oil. Pick and tear up the basil leaves and sprinkle them over the salad before serving.