When you're scarping the barrel, you're trying something sub-standard because there's nothing better. In other words, she couldn't get a better man. As for the gherkin, it means that he doesn't behave naturally, I'd guess, he's a bit stiff.
Hi, Alc112. I didn't read the book. It's one of the passages in BJ's diary which, at the conclusion of the 2001 movie, Mark Darcy 'accidentally' threw a glance at. It's really a very funny movie. I missed a lot the first time I saw it in theater; thanks to DVD subtitle feature and the PAUSE on my remote, this time I got a better grasp of what's going on I think.