This is Lesser Hawkbit, Leontodon taraxacoides. It is one of those tricky dandelion lookalikes that often grow in profusion. Telling your Hawkbits from your Hawkweeds from your Hawk's beards from your Cat's ears can be confusing. Certainly I still have to look them up each year after getting in a muddle.
Apparently they're called 'hawkbits' because of a medieval belief that hawks ate them to improve their eyesight. Silly medieval people!