If it asks for a serial number it is not the CC version of Lightroom but the perpetual license version.
On your Yosemite installation, uninstall Lightroom. Load the Adobe Creative Cloud app. Sign it out. Sign back in using your Adobe Creative Cloud ID. LR should show up. Install it via the App.
Many thanks for your reply.
Just to make it clear - I have not installed Lightroom 5 on the Yosemite system, it is only on the other hardrive and Mavericks. I have checked what is on Yosemite releating to Lightroom - there are 2 files for L5, one is in System - Library - App Support - Adobe, this file is a serial no. reference, the other is in User - Library - App Support - Adobe, these are a set of Folders for Presets, Sets & galleries etc..
From the CCloud loader Lightroom 5 will not allow Uninstall, there is nothing (program) to uninstall - it has occurred to remove the files I have mentioned to another drive and see if CCloud loader finally acknowledges it is not on the Yosemite system.
It does seem that these files are indicating to the CC loader prog that it is on Yosemite - so, if I removed the files - more particularly the serial number file - logged out of CCloud, logged back in and hope Lightroom is available to download. Do I continue if the download is for trial/serial number version - and then put back the serial number file to have it usable?
Regards Adrian CB