I may be reaching here, but the phrase "to turn left" usually means something has taken an unexpected course. In the beginning Howard and Clegg compare window cleaners, Howards is more expensive, but he lives on the edge, he has everything neatly figured out. Or so he thinks.
Pearl is his wife, as she says she's the one he runs to with a tummy ache or congestion. He can justify to himself the "expense" of Marina, she's his escape from reality, not just the relationship, but how he sees himself. When suddenly Pearl pulls a "Marina" the balance of nature is turned upside down, reality and dream come crashing down on Howard. For one, he's afraid he has lost the person who he depends on taking care of him when he's not his best, also, he knows (since we see nothing ever gets accomplished with Marina) if he were expected to be who he likes to pretend he is with Marina full time with Pearl, he'd never make it.
