Well, if anyone refers to this in future, it appears that the Linux builds we are using have integrated Firewalls (which are enabled) it seems that this (along with some SNMP.conf settings) was the main part of the issue.
Once these were amended, communication could take place.
We also did the following:
Updated Net SNMP to a more current release (that is supported on the Linux builds)
Amended the SNMP.conf files
Still working out the various minutiae of monitoring, but we're getting there 
Big thanks to Erickson from Solarwinds support for his assistance in getting this sorted!