This game is now failing to install updates and the option to re-install/repair is greyed out. It also showing as 'Not Authorised''. It use to work OK so what is the problem and how do i fix it?

Buy this :
https://www.gog.com/game/the_witcher_3_wild_hunt_free_dlc_program
then the update should work .

From GOG

Contact gog support by using the support button on the website. Tell them when you brought the game directyly from them. It's quite likely a bug as a lot of people brought fraudulent copies off sites like g2a and so they get revoked when found out. If you brought it from here support will sort you out quick enough. Also, you should still be able to play your game, and download the files directly from the website.

Thanks seems to have sdone the trick