Ok, checked the throttle cable. It does use the correct HLY 20-7 (https://www.summitracing.com/int/parts/hly-20-7/media/instructions) adapter btw:



Unhooked the cable but no: the throttle won't close any further. So not a mechanical problem at least not related to the throttle cable etc.

Checked the little pickup lever on the other side which does unscrew:



Nothing to adjust there either. It's held against a rounded rectangle by the screw so there's only one way it can be fitted (apart from flipping it over and installing it 180 DEG out).

That seems to bring me back to the throttle position sensor. If I can't adjust it to read 30 DEG with the throttle closed (as per specs) this would suggest I'll probably have to replace it.