OK, so I've finally managed to do this.
Don't know if this will help anyone or if I'm stating the obvious (although it wasn't obvious to me), but:
When you do this, you have to do it on the phone, and also crucially, it has to be done on the Google Chrome browser on Android phones, or the Safari browser on IPhones. I've no idea what browser I was using on my Android phone, but when I downloaded the Google Chrome browser, it worked straight away.