According to Coflein, the Map Ref for this site is a bit out. It should be:
SJ22131736
http://www.coflein.gov.uk/en/site/306992/details/COLLFRYN%2C+ENCLOSURE/
It also appears to be genuinely prehistoric.
That's quite confused. I've changed the grid reference, relabelled it an enclosure, and removed the post about the cursus. Thanks, TMA Ed.