You're right of course, when treating it as a noun it is two separate words. In a confused sort of way, by hyphenating I was trying to use it a a verb, in a form to suggest that spellchecking hadn't been done. Rather than the imperative "spellcheck". As it was after the event an imperative was pointless.

Spellchecking is of course something that happens at Hogwarts.

Why isn't the verb "To spellingcheck" rather than "To spellcheck"? Perhaps a spelling mistake to do with Americanisms?

You're right.... well spotted