Sorry for the delayed response. I can't even remember how the problem was resolved but it has just happened again.
Looking at the dates, both times were just after I changed to a different router (Firstly BT~Virgin and now Virgin~TendaMesh). The problem seems to be that the IdPi access key seems to remain associated with the previous router, hence the error report saying there were two addresses - when there were not.
My post now is to record that the change of router caused the problem both times and also to advise that got around the problem this time by connecting the IpPi by using the IdPi's IP-address instead of using the IdPi's access key. This proved just as easy and more importantly, it worked.
For the benefit of anyone as computer-illiterate as me, you can find the IP address by looking at the devices connected to your router. In my case, simply by looking at my router's app.
