As you will see from this link, it should be Portakabin. It's a brand name and it's a trade mark. Having said that, you will see it written without a capital letter and with a "c" instead of a "k" because, in the UK at least, it has become a generic name for the type of structure. Legally, though, the capitalised version with a "k" should be used. It should always be written as a single word.
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