Ive just got to this point in the NCR missions
(http://www.youtube.com/watch?v=0UmhXMrO3QE)

And Ranger Grant only ever says good bye, and if I fast travel anywhere I get quest failed and the NCR start shooting at me and I loose allllll the good work I've done for them....

Reallllllly annoyed, its taken 14 hours to get to this point according to steam, due to other bugs aswell during the misson. Really not happy :@

Anyone know how to fix this?

Since I had the same bug and run across this post, I'll put the resolution here so people don't have to search as much as I did. Open up a command dialog by hitting the "~" key. click on Ranger Grant and type "disable" without the quotes. If it works he disappears. Then type "enable" he comes back and auto dialogs. Happy Legion fragging!