Have you tried some facet joint tissue techniques? for the headache try the following releases: Superior oblique, both rectus capitis, oblique capitis and SCM. If those dont relieve the headache then assess the ROM of same side big toe for extension if ROM there is limited then there is you problem.
Also check pelvis in anterior pelvic tilt the SCM tends to stiffen up to balance C spine and head allignment.
Check ROM for rotation at that level if resctrited try rotares muscle releases
Manipulation is not helping since the problem is at the tissue you can perform manips as many and as long as you want it wont solve the problem cause the tissue dont let go. Possible damage from constant manips can occur deppends the age may be bad and then handling technique.
Techniques wise you need to tell me what you know till now to suggest you further handling.