Complex sentence: A tertiary candidate was Saudi intelligence, on the presumption that there might be something harmful to the Saud family in the manuscript for House of Saud, but of course Holden had left the manuscript safely stowed with his wife in England.
Split into simpler sentences: A tertiary candidate was Saudi intelligence, on the presumption that there might be something harmful to the Saud family in the manuscript for House of Saud,. But of course Holden had left the manuscript safely stowed with his wife in England.