Mr X,
Start by verifying all servers have the latest BIOS and ESM firmware installed. You stated that the server agent is 4.51, but different models of servers do not support the same version of the agent. If you install Server Administrator 1.4 on each managed server, the correct agent version should install by default. Also, verify that the Dell Server agent and Dell server agent monitoring service are running in Services on the managed servers.
You may also want to verify that ports 161 and 162 are not blocked by the router.
Another good source of information on Open Manage software can be found in the May addition of Dell Power Solutions magazine, http://www.dell.com/us/en/esg/topics/power_power.htm.

Dell Forum Administrator,
Carrie S.

Oh err - I didn't realise there were different versions of the Server agents, depending on model type -  is  there a definative list anywhere ? 

There is no definative list as such + it also depends on the operating system.
Check the most recently available in the file downloads library for your system.

As a rough guide:

x100, x200 series poweredge servers stop at HIP 3.52 (NT 4 and windows 2000)
x300 series stop at server administrator 1.33 for NT 4(agent 4.8) and Server Administartor 1.4 for Windows 200x (agent 4.9)
x400, x500, x600 series are currently at server administrator 1.5 (agent 4.100 - displayed as 4.10)

Server administrator and dell agents are available for Netware and Linux as well