Ah, Magic Fields is a fork of Flutter. Flutter I know (and dislike), and as far as I know it doesn't conflict at all with Relevanssi - after all, the text is there in the body of the post.
Actually, now that you have the relevancy scores displayed, it's starting to clear up. As you can see, the test page scores highly for David Mitchell and comes up first as it should. The Oxford debate ranks on the page two, because most items before it have "David" in the title and that makes them rank higher. Changing title boost closer to 1 might raise it higher, but it might still be behind some of those entries with word "david" many times in them ("mitchell" is rarer and more valuable than "david", so the entry should at least get on the first page).
There's no extra score awarded for having both words close to each other. That's something that would be clever and useful, give a boost for words appearing with n words of each other and so - maybe I'll investigate that in the future. It's just more complicated than just counting matches and I'm already worried Relevanssi isn't as fast and well-optimized as it should be
I think the best solution here would be to use AND operator, which makes sure both words appear. There's the problem of having no results when using AND - that sure sounds like a bug. Could you please try moving "David Mitchell" in the Oxford event entry so the words aren't right after the bullet point and then try searching with "AND" operator? I'd like to see if that has any effect.
If that continues to be a problem, you could try adjusting the title boost to, say, 2.
By the way, every time I visit your site, I end up clicking "Last" in the search results when I actually want to go the next page. I'd recommend some UI changes there... Maybe give the Next/Last buttons names instead of just arrows?