Followed up by phone (not chat) with Telstra support team. The problem was escalated and now appears to be fixed. One thing I hadn't realised was that I couldn't remain logged in to different devices at the same time - although this wasn't the actual problem. For the moment, I am now happy except for the message that the tech team needs 'specialists' to deal with Apple devices so I have signed up to Platinum membership. This (in hindsight) seems a bit rich given that so many people use Apple devices and Telstra should be able to deal with these as a matter of common procedure. However, for the moment, I have no problems. Thank you for getting back to me.
Helen