Which popular British comedy series often included the catchphrase "I have a cunning plan"?

"I have a cunning plan" is the catchphrase of Baldrick, the bumbling sidekick of the eponymous hero Blackadder in the hugely popular British comedy series "Blackadder". He is portrayed by the actor Tony Robinson (born 1946) who is also a highly regarded documentary maker, especially of matters connected to railways, archaeology, and military history. Baldrick's plans, needless to say, are often anything but cunning, and involve such bizarre matters as attempting to marry the eponymous hero Edmund Blackadder, and smuggling himself into the royal household as a pet.

The show ran from 1983-1989, though there have been subsequent specials and one-offs. The first series, incidentally spelt as "The Black Adder" (two separate words) was set during the reign of the fictional Richard IV; the second, "Black Adder II" in the times of Elizabeth I, followed by "Blackadder the Third", concentrating on the reign of George III, and "Blackadder Goes Forth", set during the First World War.

The programme is a hugely entertaining mixture of historical fact and entertaining nonsense, encompassing both slapstick comedy and the great poignancy of the final scene of "Blackadder Goes Forth", where Baldrick, Blackadder, and their companions go over the top to their deaths and the screen transforms into a field of poppies.