The udp monitor is a bit counterintuitive in that it considers a service UP if it receives no response from the node. If there is any sort of response it assumes it to be an error and therefore marks the service down.
So pretty much the only option for effectively monitoring this type of service is to call an external monitor script that can interactively check the application.
There doesn't appear to be anything in the codeshare for NTP, but there is this script for DNS, which is along the same lines as a udp service:
Click here
Denny