UPDATE -
Hi Again All, I figured it out ....
Under the Proxy settings for the site/server you want to forward.....
Click on edit of the desired proxy host,
Click on the Advance tab
In the box insert the following line - location = /{return 301 $scheme://$http_host/cacti;} OR
location = /{return 301 $scheme://$https_host/cacti;}
"Replace the part in red with your desired sub folder"
Example
if your server is "192.168.29.100:2020/cacti" if placed under the details tab as is you will end up with invalid and non loading URL, So for this to work place the following line above under the advance tab to get the following.
Http:// or Https://mydomain.co.uk/cacti.
If Requested I can write a complete guide to this