Thanks for the input. Nothing obvious in the logfiles for the switch in question - it seemed to be running OK: 2014-10-13 17:17:23,350 [INFO schedule.netboxjobscheduler] [snmpcheck library.keb] snmpcheck for library.keb completed in 0:00:00.004969. next run in 0:29:59.995051. (however, only this check and three DNS checks on each loop)
However, I just rebooted the NAV appliance and suddenly after a reboot, all is well and it's talking to the SNMP agent on the switch again, and logging more results than above. I am pretty sure I had rebooted it before which hadn't fixed it, but can't be quite certain whether the last reboot was before or after it broke - but I assume that's the reason I rebooted!
So bit of a mystery why it had stopped working and why it had started again. It clearly wasn't anything up with the network or the configuration of either NAV or the switch as nothing has changed on either.
Have also now configured it to be able to access the Internet to install tcpdump etc so will do that in case it breaks again!
Steve.