If the cable was put in when the shifter was in a middle position (rather than at the smallest-cog position) then it might be hitting a stop somehow. Seems unlikely and the indicator window should tell you if that's happening.
I took it apart, cleaned and regreased it all and it's feeling less notchy at least, though I have no idea if it has fixed anything. Thanks for the suggestions, the weekend will tell whether it worked or not!
If the cable was put in when the shifter was in a middle position (rather than at the smallest-cog position) then it might be hitting a stop somehow. Seems unlikely and the indicator window should tell you if that's happening.