This is Hieracium radyrense, or Rady Hawkweed. I am indebted to Nigel Keene for pointing out that this very rare plant is only known to exist in and around the location of the old Radyr Quarry. This plant is quite likely to still be in flower this month, so if you have seen one in or around the site in question (the woods or the fields), please do take a photograph and make a note of exactly where it is (and let us know!).
The plant has been seen in the past about 20 metres from the site, mainly on the other side of the railway line. If you want to read more, you can find an interesting article here.