I'm afraid you will be out of luck, as Archibald Duncan's 2002 book
quoted earlier has been influential on this question - see for instance,
Alice Taylor in 'Robert de Londres, Illegitimate Son of William, King of
Scots, c.1170–1225', *Haskins Society Journal* 19 (2007), p. 102:
"Another Competitor in 1291, Roger de Mandeville, claimed that his
great-grandmother had been Africa, daughter of William I. Roger stated
that Africa had been one of three children, two of them daughters, all
of whom were William's. The son and one daughter were murdered by
Malcolm, brother of 'King' William, but Africa was spared and escaped to
Ulster where she married William de Say. The story makes no
chronological sense: Malcolm, the eldest brother, was king of the Scots
between 1153 and 1165 and thus dead before William became king.
Professor Archie Duncan has accordingly drawn attention to the fabulous
nature of this claim and demonstrated that Africa does not appear in any
deed of the de Says. The relative abundance of evidence about William's
other illegitimate children suggests that the silence of contemporary
sources indicates that the existence of Henry and Africa was fabricated
by claimants to the kingship of the Scots over a century later."
and Susan Marshall in *Illegitimacy in Medieval Scotland, 1100-1500*,
The Scottish Historical Review Monograph, second series 3 (Woodbridge,
2021), p. 85:
"Two of the claimants, Patrick Galightly, a burgess of Perth, and Roger
de Mandeville, of an Anglo-French family settled in Ulster, claimed
descent from two otherwise unknown children of William the Lion - a son,
Henry, and a daughter, Affrica, respectively. Affrica was said to have
married into the de Say family, but is not mentioned in any surviving
records concerned with them; the number of generations between her and
her alleged descendant Roger de Mandeville makes it implausible that she
was born of William; and the Mandeville claim is accompanied by a
fantastical tale alleging that Affrica had had two siblings, also the
offspring of William the Lion, a brother having been murdered by his
uncle (impossibly, William's brother Mael Coluim IV), and a sister
having died subsequently. While William the Lion's illegitimate
son Robert of London frequently witnessed his father's charters, and he
and his illegitimate half-sisters Isabella, Margaret and Ada are all
found in other contemporary sources, no Henry or Affrica who were
children of William the Lion appear in any charter or other record.
Duncan and Taylor both contend that Affrica and Henry were invented by
the 1291 claimants to provide grounds for their claims."
Once rejecting the alleged paternity of Affrica, Scottish historiography
has little interest in anyone between her (assuming she even existed)
and the uncompetitive claimant Roger.