Innit is used as a general-purpose placeholder to stand in for the appropriate form of a negative-question suffix. For example:

We're going to the pictures, aren't we? -> We're going to the pictures, innit?

That was really good, wasn't it? -> That was well wicked, innit?

This comes from English requiring specific forms of the negative question. In Swedish, one can say 'eller hur?' ('or how?'), and in French, 'n'est-ce-pas' (literally, 'isn't it?'), no matter what the verb, tense or number of the preceding phrase. Speakers of languages such as Welsh, and numerous Indian languages (which have links to Welsh, incidentally) naturally adapt this usage to English. While this is probably a Good Thing and part of the development of the language, it's confusing right now. Gritchka informs me that the usage is principally Cockney, but of Indian/Welsh origin.