Further to my post above. After some more hours I managed to get it working. In the end all I needed was this line:
sudo iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-port 8123
The issue (as it turns out) was the port forwarding rule on my router .
Once that was resolved and after I added the iptables rule I installed a package called iptables-persistent which deals with making the rule persist after reboots.
sudo apt-get install iptables-persistent
Hopefully this helps someone.