J&C Bord say that Whiteleaf Cross has not been dated, but that a local boundary mark called ‘Wayland’s Stock’ was mentioned in a charter of 903AD, and perhaps they’re one and the same. ‘Stock’ could be interpreted as alluding to something stick-like (which rather ties in with the hypothesis that the cross is actually a christianised phallic symbol). But perhaps that’s just finding what you want to / expect to find.
(info in their ‘Atlas of Magical Britain’, but no more details about what charter it was from)