A credible newspaper should do some fact checking before printing what 'sources say', otherwise its just PR for whatever wants to get this published.

Person A: The Guardian is great. Their "quality is consistently high."

Person B: The Guardian tells lies about relatively important stories like Assange and Manafort meeting.

Person C (you): Actually, all UK print media lies!

Is this a fair characterization of what just happened?

No.

The Guardian has not lied. The Guardian has accurately reported what sources said. This is what all UK print media does. Guardian is somewhat better than some other UK print media because they'll (as they did here) include the words "sources say" in the headline or sub head, or they'll include quote marks in the headline.

Okay, this seems like a bad faith argument to me. You must be aware that print media in the UK, including The Guardian, also publish stories that are backed by real evidence (e.g. photos, videos, people openly stating, "I personally did/said that", etc.) rather than mysterious, unnamed sources.

Plus, it looks disingenuous to pretend that The Guardian didn't willfully choose to print this story instead of any of the many many stories about Barack Obama being a "secret Muslim" or even "gay" according to some source.

You can further clarify why you think The Guardian is actually good if you want, but if it involves, "Everyone else publishes stories from sources," I'm done taking it seriously.