Fixed it myself, sorry!
It wasn’t my VS error that was the issue, though I would like to know why I am getting that.
I was also getting some errors about other standard assets - namely Cross Platform Input. I had only imported the Utility section. I have now imported the whole standard assets package, those errors went away on a restart of Unity and I can assign the script to my game object.
Woohoo! Funny how asking for help on a forum helps to help yourself sometime.
Maybe this will be useful for someone else in the future…