a real world point of view
A line producer is, in most production teams, the person between the producer and the heads of department. She is "the producer that does all the work" in the sense that he or she must be in daily contact with all departments to ensure satisfactory progress is made. Line producers are usually responsible for the creation of a budget and are at least in titular charge of the payment of the below the line crew. These responsibilities give the line producer added significance to all but the most senior (or above the line) personnel. Line producers are almost always required throughout both pre-production and principal photography.
Most productions — including DWU shows in the BBC Wales era — tend not to involve line producers in the affairs of post-production departments, which generally have strong and specialised post-production management.
On occasion, line producers may take greater responsibilities and actually have creative input into an episode. When this occurs — as with Patrick Schweitzer on The Vampires of Venice and Vincent and the Doctor — a line producer can be credited as a an additional producer or a co-producer.
The title has rarely been used in DWU history, and was only regularly applied to Patrick Schweitzer during series 5. Otherwise, it has been used on episodes with a significant overseas crew, as happened with the British-produced TV: The Fires of Pompeii (where the line producer was Italian Guido Cerasuolo) and the American-made TV: The New World (where the line producer was Briton Debbi Slater).