Hi Laura
There was a time I think that steering balls had to be notified to DVLA and you must have the code 40 (modified steering) to use one however I’ve been told since you don’t. Years ago many abled drivers used steering ball before the event of power steering in particular HGV drivers but their rarer these days.
I assume we as disabled drivers with our particular problems must have the ball and the code to enable us to drive however if your abled bodied you can use a steering ball without the code on your license but again, so I’ve been told and read.
Just a bit of info, did you know that steering balls are also known as suicide balls or suicide spinners as they can be dangerous if your unaware of oversteer and understeer that a steering ball can easily bring.