Following my removal of the security update Windows reinstalled it the following night and the selection tool problem returned. I uninstalled it again, which solved the problem again. Windows then fought back and reinstalled the troublesome update...but this time sketchup is still running fine!
The update has the same code and I haven't updated my graphics driver or (knowingly) made any other changes. So, it's a bit of a mystery, but at least I can stop fighting my OS and get on with my work now!
For the record, I'm running 2016 Pro.