I go to Manchester as a CS student but did not have an interview, however I can tell you what I've heard from other students which did have an interview.
The interview is mostly a show-around, lecture, a Q&A with current students, and an informal interview. It's nothing to worry about, as long as you can answer questions like the ones Strangey posted then you will be fine.
The point you said about interview = offer is not always true, but is often true.
If you have any questions about CS at Manchester which you would like answered then I'd be happy to answer them.