As I anticipated my Mac problems have been resolved by Presonus support informing me that they have moved the Studio One content downloads to a higher priority location yesterday.
That declared action must have been implemented because I am now able to obtain all the 36GB of Studio One 3 content for my iMac.
For detail of issues experienced see on this forum:
Are content download problems for Mac’s recognised and what active steps are being taken to eradicate it ?
TonyGB