Thanks for getting back to me so quickly. I can't say whether this is a new error or not as this is the first time that I've built a course using labels as shortcuts to activities. I'm afraid I can't send a backup of the course as it is emblazoned with the clients details so I am not permitted to share.
The strange thing is that I am able to view external sites, documents, videos from a label with a hyperlink. It is only when it comes to accessing a quiz via a hyperlink when it fails - but only with the iOS app. The Android app works like a charm. So the question is what is the iOS app doing differently?
BTW I have just installed the new iOS app 3.5.0 (version code 3501)