People - thanks for taking the time to reply. After my post I returned the Hub 3 to router mode and deliberately walked away from the issue for 24 hours or so. Sunday when my team went 3-0 down I decided to re-investigate the issue and by chance stumbled on varying similar lines of action to those mentioned here. I would like to point out that the root cause of my issue was my Hub 3 indicating it was in Modem mode when it wasn't - powering down and up on more than once occasion by fluke actually placed the Hub into Modem Mode (LAN port 1 only providing internet access, Red / Magenta LED ON, access to the config menu via address 198.168.100.1)
My fix was this -
Step 1 - Hub 3 in router mode all LAN ports able to access the internet (Mac connected to LAN port 1). No cycling of power of the Hub
Step 2 - I disconnected the Mac from the Hub and and connected LAN port 1 of the main Deco S4 unit into LAN port 1 of the Hub 3 LAN and powered up the main Deco
Step 3 - Connected the Mac into the LAN port 2 of the main Deco
Step 4 - Used the Deco app on my iPhone to register the main Deco - Default configuration on the app for the Deco S4 used. Mac able to surf the internet.
Step 5 - Connected the Mac back into LAN port 2 on the Hub and placed it into Modem mode - no other changes made to the Hub 3 configuration.
Step 6 - Disconnected the Mac from the Hub and re-connected back into LAN port 2 of the Deco
Step 7 - Allowed the Hub to change into Modem mode. On receiving a steady Red/Magenta LED I cycled power and waited until once again I received a steady Red/Magenta LED.
Step 8 - Main Deco and the Mac connected to it will not connect to the internet.
Step 9 - Cycled the power to the Hub 3 once again and waited for a steady Red / Magenta LED - STILL Main Deco and the Mac connected to it will not connect to the internet.
Step 10 - Cycled power to the main Deco -STILL Main Deco and the Mac connected to it will not connect to the internet.
Step 10 - Getting annoyed and by chance repeated step 9 and this time - Mac connected to the internet.
It's possible I may not of cycled power to the Hub and/or the main Deco in the correct sequence but I'm now good thanks and have no problem with anyone pointing to the error of my ways. Satellite Deco units connected in minutes and WiFi dead spots eliminated.