How to renew Safari certificates?

For a couple of months I have had troubles with the Guardian website. Pictures would not but would be replaced by question marks in little blue boxes. I selected "always trust" in the certificate drop down but didn't help. Re-started the computer. Sometimes helped for ten minutes or so. Finally got so annoyed I found the certificates for the Guardian website and deleted them with the idea that this would probably force some sort of update or certificate refresh that might fix the problem. Alas now I can't open the Guardian website at all - "Safari Can't Open the Page" because unable to establish secure connection. Any ideas how to fix this?

Macbook Pro retina 15 inch late 2013

Safari 10.0.1 (latest)

macOS 10.12.1 (latest)