uninstall everything cc including lastly the cc desktop app, https://helpx.adobe.com/creative-cloud/help/uninstall-creative-cloud-desktop-app.html
clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html
restart your computer (don't skip this)
reinstall the cc desktop app, https://creative.adobe.com/products/creative-cloud using compatibility mode (https://technet.microsoft.com/en-us/library/bb456979.aspx?f=255&MSPPError=-2147217396)
use the desktop app to install your cc programs/trials
Thanks for your help.
Sadly, I have followed the above instructions a number of times (including shutting down the mac at appropriate times) and no good.
Each time it says it is going to launch lightroom it goes back into the same circular hell!
Oh well, trying to move away from Apple's Aperture onto Adobe is obviously not such a good idea for me!
using a browser that allows popups and cookies, contact adobe support during pst business hours by clicking here and, when available, click 'still need help', http://helpx.adobe.com/x-productkb/global/service-ccm.html
Thanks, called support today, they were very helpful and resolved it with about 20 minutes on the phone, Peter