[Perkin]

I opened the epub in sigil and done a search-count for quotes just in the first chapter - was 79, so a quote is missing from a paragraph somewhere (or from several).
(just because there may be an even number, doesn't guarantee they're matching pairs)

Use Sigil, and do a search for a quote mark, then just keep looking for the next one, and try to spot any sentences where one is missing, either from the beginning or end of speech/quote etc., then when you find one missing, add it, continue till you're happy.

Edit:
So continue through rest of book.