Ereira worked at the BBC on television and radio since 1965 contributing documentaries to the Timewatch strand amongst others.
He has been awarded the Japan Prize for his 1978 documentary on the Battle of the Somme, and the Royal Television Society Best Documentary Series award for his 1988 documentary on the Armada.
In his books The Heart of the World (1990) and The Elder Brothers (1992), he documented his visits to the Kogi people of Colombia, an indigenous ('Indian') ethnic group which survived attempts by the Spanish conquerors to destroy them by retreating high up in the mountainous area of the Sierra Nevada where they live. Ereira regards the Kogi as unique amongst indigenous peoples in the Americas in that they have managed to retain their traditional culture almost entirely intact. Since the 1980s, the Kogi have warned, on the basis of their observation of ecological changes in the Sierra Nevada, that the world is facing an ecological catastrophe. They asked Ereira to make his films about them in order to warn the rest of the world (and particularly the West) that it needs to radically change its way of living, and its exploitative attitude to the natural world, if it is to avert this catastrophe.
Ereira collaborated with Terry Jones on the documentary series Crusades (1995), Terry Jones' Medieval Lives (2004) and Terry Jones' Barbarians (2006), with whom he also co-authored the respective companion books.
In 2004 he also presented, wrote and produced a 6-part documentary series on the kings and queens of England for UKTV History.