That is either Scentless Mayweed, Tripleurospermum inodorum, or Scented Mayweed, Matricaria recutita. The latter is the most common source of chamomile. To tell the difference you need to rub it and see if it is aromatic or cut a flower in half. See link for the differences.

Here is a link that might be useful: MAyweeds

thanks flora can I ask what would I be looking for if I cut the flower in half? please