Many thanks tgriep for all your support as your diagnostics of the issue were correct.
I have started to manually reload the mibs.
What it means is that all the mibs land up in a single snmptt file. Not ideal, as I wanted to run separate files for different manufacturers as it allows changes to be made easily rather than digging through a large file to make changes, but at least now the alarms are coming through.
I am now seeing Critical, Warning and Normal trap alarms coming through to Nagios.
Many thanks !!