Do I need a licence to let one flat?

Sometimes. It depends on the street, not on the property.

A single flat let to one household is not an HMO, so mandatory licensing does not apply. But many councils run selective licensing, which covers every private let in a designated area regardless of who lives there.

The designated areas change, and they are decided by the council rather than by anything about your building. Check your council’s licensing map before you assume, because the fine for getting it wrong is not small.