How to change Redmine port from 80

We just upgraded our old Redmine box to Jumpbox, however now Redmine is running on port 80 where as before it ran on port 3000. (Jumpbox admin now uses port 3000)

Port 80 and 443 are no good as it means we can't access Redmine externally due to having other services running on those ports.

I've tried modifying the /etc/apache2/ports.conf however this just kills it completely...

Help please :)

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