"Of course, "sredir" should be defined in the services file (usually /etc/services, usual values start at port 7000)"
So, if you are using inetd, you can select the port to use by modifying /etc/services. If you are using xinetd, take a look at sercd.xinetd instead.
I used netstat to determine which port it was listening on but that didn't tell me anything. It looks like sercd isn't listening on a port so how does it communicate over the network?
It's inetd/xinetd that listens on the port.