Thanks for your help falko
In case it's of use to others, here's what I did to change the port to 51.
1. Add the new port to System Firewall in the control panel. Don't forget to Save.
2. Check with nmap that the new port is available (closed).
3. Edit three lines in /etc/apache2/sites-available/ispconfig.vhost
4. restart apache2
5. Find the control panel at the new port.
After completing the procedure up to here I got a bit worried that in Monitor it showed up in red that the web server was offline. However this problem disappeared after a few minutes and everything is now working fine. Maybe the control panel takes time to update after the apache2 restart.
6. Remove the old port from System Firewall in the control panel. Don't forget to Save.
At this stage I was a bit concerned to see a popup message "Sorry there was an Error"
Closing and restarting the browser access to the new port cured that.
7. Check with nmap that only needed ports are open.
8. Check that the services are working correctly at the new ports.
Comments on this procedure are welcome, particularly if I'm likely to find anything else that was broken as a result of doing it.