Therefore, its known narrative elements are not a part of the Doctor Who universe. It may have been the basis for a similar story in another medium, however — and that story may indeed be a part of the DWU.
Hex was a Doctor Who TV serial written by Peter Ling and Hazel Adair for Season 21. It would have featured the Fifth Doctor and Peri Brown. It was eventually adapted by Paul Finch into the audio story Hexagora.