Just off the wires, David Tennant will assume the hosting duties of Masterpiece 'megaprogram' in the fall. I call it mega program because masterpiece Theater now encampasses Mystery!, Masterpiece Classic, as well as Masterpiece Theater.
WGBH in Boston produces all these programs, or contributes to the production to an extent. It seems natural that Tennant would become host since a lot of the recent broadcasts have had a lot of Who actors in them.
Hosting duties pretty much just entail topping and tailing the broadcast. Here in the Chicagoland area, Masterpiece is shown on Sunday evenings. With WTTW Prime, the digital 11.2 channel showing multiple screenings during the week of Masterpiece Programming.