There is actually considerable debate as to who made the compilation. Many manuscripts ascribe the the history to Gildas (Biography) another monkish historian (or possibly two Gildases*), while the manuscript from the Vatican is ascribed to "Mark the Hermit, a British Bishop"*. You can read a detailed discussion of the arguments about the authorship by Robert Vermaat in his excellent summary at Vortigern Studies, or in Dumville* vs. Field*, or, in the case of Mark, Newell*

Gildas' venomous De excidio Britann(i)ae liber querulus ("the Fall/Ruin/Capture of Britain, a whining book": see Other Works), written c.545 CE * , is regarded as one of the more reliable accounts of the age. It is a gloriously tirade against the sinful uselessness of the British. A life of St.Germanus, written by Constantius Hericus mentions a British Bishop Mark as giving the information which is also in the Historia in 876/7 CE*