Thanks for the replies on this.
as a quick update, inner and outer cable replaced and no different. Managed to source a new second hand gx mech at a reasonable price so fitted that today and sure enough shifting back to 100 percent.
When it works 12 speed GX is lovely but I feel that a year and not a massive amount of miles out of a mech is pretty poor. Admittedly it took a very slight knock but I don’t feel enough of one to cause issues.
I would snap up a similar gx /XT quality decent range (say 10-50) 10 speed set up that isn’t so picky with indexing.
Maybe link glide is the answer but I do wonder about the strength of the cage on the mech having had an SLX one if the link glide ones are no beefier.