I'd previously prepared the drive using Gparted and then at install used the 'Something Else' option ensuring each partition's flag was set appropriately. I've more recently done exactly the same thing installing another distro on another hdd without any problem and did not have a problem initially installing Z16.
Anyway.. have come to a decision about this situation tonight after.. well read below.. 
Btw excuse my little text format errors - it's a while since I was on a forum and I should know better as I was Admin on some for years(!)..
Anyways..
@Aravisian
Well.. I followed your advice to take ownership of .root & /home after first sifting through files on 'both' /home partitions(!) They were well messed up showing that Zorin had used both to store system files.. which gave me a tedious job for an hour or so sorting them out.
After this I used your commands in the terminal and checking saw those commands successfuly gave me ownership while I swapped files around.
Then the real problems started 
because now the system could not find 'anything' ! ..and I even had a black screen but could still just about see things..
I'd also not backed my files up since 2 days ago so there were some files that I didn't want to lose and I should have backed them up.. but in my haste forgot(!)..
So then I need to try to 'undo' the changes - not an easy task....
Having no access to anything in the Home directory I had to work from the terminal to try to fix things. So used these commands to get me back to where I started from:-
sudo chown -R who:who /home/who/
find /home/who/ -type f -print0 | xargs -0 sudo chmod -Rv 644
find /home/who/ -type d -print0 | xargs -0 sudo chmod -Rv 755
sudo chown -R who:who /media/who/_home/who
(Not sure how to put the commands in neat little boxes as you did!)
This got me part way back to normality. At least I could access files again and remove the hundreds of locked files(!)..
Had to do a bit more fiddling about using DISKS and the Terminal but now I'm back to how things were before (more or less!)
So my decision now is to do a clean re-install because it was aparent a lot more needed doing to swap those /home directory contents over whilst also keeping permissions easily managable..
Still.. I learnt some new commands in the process..heh heh
Thanks to all for the help.. 
oh and I can spell 'apparent too'..
Hi again guys..
Had to just add this update before I go off to bed..
After leaving the computer alone for an hour or so and relaxing I went back to it to have another go.. as is my usual way.. and unbelievably 'fixed it' in about 5 minutes !
This time no Terminal commands.. I simply opened DISKS and deleted the mount point for my /home partition that wasn't being used. Then edited the mount point as /home in the hope Zorin would start to use it again.. and that was almost all I needed to do !
This time I got no error pop-ups telling me something about 'quark' (wasn't he in Star-Trek ?).. Anyway.. I rebooted and was pleased to see that now my Home directory was indeed being used as such
There were a few hiccups I then had to sort out. Black screen came back and Zorin froze at one point.. but after a few more tweaks and another reboot that was fixed too. Very funny that Zorin has gone back in time.. 
I know this is due to some older files being in the /home folder before Zorin decided to make a duplicate !
So no re-install needed and I just have to tidy up remnants of files from Apps and different Zorin states from the recent past.
Also.. previously I'd found I had lost some files and folders that were on my desktop when trying to swap the two /home folder contents &etc.. but resigned myself to them being gone for good but the nice surprice was that now they have returned to the Desktop.. (and since backed-up).. so nothing was lost.. just hidden.. somewhere..?
Thanks to all who replied in trying to help and especially for motivating me to fix this..
Tony
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