Two people cling to each other late into the night, and the poem treats that closeness as both precious and fragile. The "two pieces of thin ice" suggest that each person is brittle, or that the connection between them could crack at any moment. Yet they hold on, which gives the opening a quiet tenderness.
The strangest image is night "leasing" day "like a flat in London." A lease is temporary and costs something, so the line makes the darkness feel borrowed rather than owned. The comparison to a rented flat also gives the cosmic moment a cramped, everyday feel. The speaker seems to see their shared hours as something on loan, with an end date approaching.
The mood is intimate, wistful, and a little uneasy. Nothing dramatic happens; they simply stay together "till past one." That plain ending lets the feeling settle: this is a small stretch of time, but it matters. Judging by the imagery, the speaker knows it can't last, and values it for that reason.