Ok, it has taken me a while to get this working, so I thought I would share how I got the outcome I wanted. Guess this shows how good Resilio is, as I have had the pro version since it came out and have hardly ever had to do anything with it, so each time i do, it takes relearning all over again.
Step 1.
Unlink all devices, then re-link them all from the NAS Resilio Sync with all set to Disconnect. I had to reinstall the licence key here too as when I unlinked from my MacBook, I lost the license.
Step 2. Disconnect and delete all previous synced folders.
Step 3.
I then copied all folders from my MacBook to the NAS, using the same folder structure. This is also the same folder structure as I have on my iMac.
Step 4.
From the DSM NAS Resilio package launched from the Macbook, I created a new Standard Folder pointing to Documents on the NAS. I then copied the Read/Write Key.
Step 5.
On the MacBook I opened the MackBook Resilio window, and clicked on the +, and then on Enter a Key or Link. I pasted in the Key I had copied from the NAS Resilio Package, and the clicked Connect on the MacBook Resilio and chose my Documents folder. Agreed to the warning that the folder was not empty, and that was it for the MacBook.
Step 6.
On the iMac I opened the NAS DSM, launched the Resilio Package and copied the Read & Write key. I then opened Resilio on the iMac and clicked on the +, and then on Enter a Key or Link, as in Step 5.
I can now share selective folders to my iMac or MackBook, and have the NAS as the master repository for all files.
Not sure if this is clear enough, or whether its confusing, but hope it helps anyone who has the same issues I had.