Hi Graham,
I was just about to write to you to say it hadn't worked, but then started to look at other security settings. After having added the certificate, it seems I needed to restart Outlook (when I closed Outlook I was prompted to save the VBA project). When I restarted I got an alert about the electronic signature, and after I accepted it, it started to work again.
Thank you again for being so generous with your time and knowledge!
Paul