There is provided an adjustable fastening device configured to engage a support to provide a tether for the attachment of a tensioned line. The adjustable fastening device includes a plate, and an adjustable collar configured to extend around the support and securely engage with the plate in operation. There is also provided a tensioning mechanism configured to seat against the support and extend in length to drive the support and the plate apart in operation when the plate is securely engaged with the collar. In this way the plate is moved away from the support to thereby securely engage the collar against the support on an opposed side of the support from the tensioning mechanism. The tensioning mechanism includes a rigidly connected foot. The foot includes a surface configured for engaging with the support in operation and preventing the tensioning mechanism being unseated from the support by tension in the collar acting in a direction perpendicularly to the direction of elongation of the tensioning mechanism.