Suzanne Barnhill, Swayback's maven of English usage, has produced evidence that the British can handle the Queen's English about as roughly as their colonial cousins in America can.
She produced an article from The Telegraph, one of Great Britain's leading newspapers, that dealt with the Norman conquest of 1066. That's the invasion that started the blending of French with Anglo-Saxon to produce the English language.
One sentence read: “The extent of the constitutional changes resulting from the Conquest were fiercely debated by the Victorian historians E.A. Freeman and J.H. Round. What is certain is that it tilted the focus of the ruling class away from the northern world towards France.”
“The extent were”? Suzanne remarked. “And what is the antecedent of ‘it' in the second sentence?”
The writer obviously took his eyes off the singular subject, “extent,” and swung at the plural “changes,” which is actually the object of the preposition “of.” Since “extent” is singular, he should have written “was debated.”
Next, we have to ask what tilted the focus of the ruling class. The writer says “it” did, but doesn't identify “it.” “It” could mean that the extent of the changes tilted the focus. Or it could mean that the debate itself did the tilting.