in the Oldfield book, David explains that when they jammed together with Mark on acoustic guitars, before DS in Mark's flat at Buckhust Hill (CIRCA 75-76 I guess) they played more or less what became songs on two first albums. For example, he remembers the melodic line in Lions, and when Mark composed the song in studio in february-march 78, David gave him the idea to use this melody.
david explains that these jams were not exactly "songs" but more "a sound, a kind of style, of feeling"
so indeed, David "participated" in the DS groove so to say, but not exactly in writing songs imho