What should I bear in mind regarding my speaking pace and how I conclude my presentation at the assessment centre?

It’s better to speak a little more slowly than too quickly, and make good use of the art of the pause. Please do not end your presentation with the words: “Right, that’s it.” It goes down much better if you thank the audience for their attention.

Can the other candidates also be present during the role-play exercise at the assessment centre?

That is rather unlikely. For one thing, the tasks would have to be varied frequently enough; for another, the participants present might also draw conclusions for their own conversations from their observations.

Should I mention in the job interview that I’d like to set up my own business in the medium to long term?

If the interview is for a fixed-term position, this isn’t a problem. However, if a permanent position is on offer, you could end up ruling yourself out.

Should I state in the subject line whether I’m applying for a part-time or full-time role?

You should definitely state your preferred working hours so that the employer understands exactly what you mean. This doesn’t necessarily have to be in the subject line, unless there are several similar vacancies that differ only in terms of the number of hours.