OK, getting somewhere. I changed tack and built a 1.4 install locally in vmware to avoid all firewalls and anything unknown. I created a new engineID then reset the auth and priv passwords. Amazingly snmpwalk worked first time, though I noticed it makes a difference whether the passwords are created in quotes or not.
The difference being that in the Paessler SNMP tester and SNMPwalk GUI I was treating the engineID as the context and that’s what didn’t work. My clue was snmpwalk not needing a context. Change my query in Paessler Tester and that worked too.
Now I just need to make Solarwinds discover it and I’ll be home dry. It’s a shame Solarwinds appears to be a law unto itself when it comes to SNMP testing 
Cheers!