PostgreSQL Connection to RDS from external server - Connection errors but works from other sources

I have a Lambda Python function connecting via psycopg2 to a PostgreSQL db instance running RDS. The Lambda connects absolutely fine (Lambda and RDS both in EU-West-2 region) I can also connect to the PostgreSQL via PgAdmin4 from a local development system and other developers can also access from other locations/IPs via PGAdmin with no problem. I can also connect a simple psycopg2 connect and query script from my local desktop here. Therefore I know RDS is accepting and responding to externally-sourced psycopg2 connections and queries. HOWEVER, when I upload the same simple connect script to my web server (OVH - based in France if of any relevance), running equivalent Python and psycopg2 etc., the connection fails with the standard psycopg2 error response from the Python:

Error raised: connection to server at "xxxxxxxx.yyyyyyyyyy.eu-west-2.rds.amazonaws.com" (ppp.qqq.rrr.ssss), port 5432 failed: Connection refused Is the server running on that host and accepting TCP/IP connections?

I've tweaked the Security Group settings to permit anything from anywhere etc and still no joy. PostgreSQL in the RDS seems to have listening on * which seems necessary to permit connections under certain circumstances. What is the subtlety in the differing sources that means such a connection from the OVH web server won't work; I can't find anything in the docs that seems to link to this issue and there's nothing obvious mis-configured on the server-side.. Any responses gratefully received.