When I had my interview I was led to believe that I would be teaching certain groups. On arrival, I find myself assigned a less interesting timetable in an annexe that I didn’t know anything about. What can I do?

It depends how seriously you view the situation. At one extreme, if the advert and job description are nothing like what has materialised, you could threaten to quit. However, this is an extreme measure, and schools have the right to change timetables due to changes in circumstances. It is to protect themselves against cases like this that teachers join professional associations. The school representative should be your first contact to discuss the situation. It may be that something can be negotiated. At the least, you should be offered an explanation as to why the change has taken place. If you are genuinely not satisfied, then start looking for another job.