Thanks kkng and till but I'm still unable to change the port.
I now have two instances of ispconfig3 on different debian lenny boxes.
The first one was set up with the default port 8080 selected during installation.
The second one was set up with port 50 selected during installation.
Both were set up using the procedure at
and both are working properly as far as I can tell.
The control panel works fine at http://192.168.1.120:50
on the second box.
Now I want to change the first box from 8080 to something else, say 51.
I don't want to reinstall it because it's in use for web services.
I'm the only person who uses the control panel.
I searched the contents of these files on both boxes
It wouldn't surprise me if I misunderstood something but I cannot find 8080 or 50 in either file on either box.