Boil the kettle. Tip half the chickpeas into a food processor, add roughly half the reserved liquid from the can (the liquid should come to just below the level of the chickpeas in the blender), the tahini, olive oil, garlic, lemon zest and juice and some seasoning. Blitz until you have a smooth, loose hummus. Check for seasoning.
Cook the pasta following pack instructions. Drain, reserving a mugful of the cooking water, and rinse the under cold running water for a few seconds until cool.
Toss the cooked pasta, spinach, tomatoes, cucumbers, olives, the rest of the chickpeas and the hummus dressing together in a large bowl until everything is well-coated. Add a splash of the reserved pasta cooking water if the dressing is too thick. Will keep covered and chilled for up to 6 hrs, or in an airtight container in a cool bag for 2 hrs. Add a splash of water to loosen the dressing again before serving.