Plot thickens:
For some reason it seems to tls-verify successfully, but only for the first connection after making a change (which reloads the server config I'm guessing), subsequent connections fail as above:
openvpn[56619]: x.x.x.x:59134 VERIFY SCRIPT OK: depth=1, C=xx, ST=xx, L=xxxxx, O=xxxxx, CN=vpn.example.com, emailAddress=xxxxx