Have we mis-spelt "organisation" ?

On our Web site it says: "Organisations either tackle change themselves or they use an outside body. We are an extension to your internal resources."

You might be thinking, it's 'organization', surely?

This is one of those situations where there are two nations separated by a common language! We're based in the United Kingdom, where "organisation" is the preferred spelling. "Organization" is acceptable here as well (indeed, -ize is preferred by the OED), but most Britons, mistakenly, would see it as a misspelling.

The Merriam-Webster Online Dictionary has an entry for "organisation".

When we're working for clients, we use British English or American English spellings, according to their preference.