Barbara Covett: People like Sheba think they know what it is to be lonely. But of the drip, drip of the long-haul, no-end-in-sight solitude, they know nothing. What it's like to construct an entire weekend around a visit to the launderette. Or to be so chronically untouched that the accidental brush of a bus conductor's hand sends a jolt of longing straight to your groin. Of this, Sheba and her like have no clue.