Brits: Is the styling of this title incorrect?

I’m reading a novel that takes place in England and has several titled characters. One of them has a name something like Sir Henry Williston. Sometimes the author refers to him as “Sir Henry” and sometimes as “Sir Williston.” Wouldn’t “Sir Williston” be incorrect?

The author is an Australian-born American. I would have expected any author setting a novel in the U.K. to have done his homework on the peerage, but this looks flat wrong to me. Yes or no?