Heat the olive oil in a wide pan over a medium heat and add the garlic, parsley and chilli flakes. Heat, stirring, for one minute, until the garlic starts to colour.
Add the stock and half of the tomatoes and bring to the boil.
Add the fregola pasta and simmer for ten minutes (or to cook according to packet instructions), stirring frequently, until the pasta is just cooked.
Meanwhile, pour enough water into a large clean pan with a lid to come about ½cm/¼in up the sides. Bring to the boil, then add the clams and cover tightly with the lid. Cook, shaking the pan occasionally, over a high heat, for 3-5 minutes, or until all of the clams have opened.
Remove the clams from the pan with a slotted spoon. Discard any clams that have not opened.
Carefully pour off most of the cooking liquor from the pan into a bowl, leaving behind any sand and grit that will have settled on the bottom of the pan.
Add the clams, the strained cooking liquor, the infused saffron and its water, the lemon zest and the remaining tomatoes to the pan of simmering pasta and mix well.
Remove from the heat and ladle into six soup bowls. Drizzle with a little olive oil and serve immediately.