As an agency worker, even if you're on a long-term placement with a company, there's no obligation on that company to offer you a job directly employed by them.
However, agency workers have the right to be informed about vacancies with the hirer while on assignment with them. The aim is to give you the same access to new permanent vacancies as your directly employed co-workers.
The hirer can choose how to inform staff about the vacancies. Whether this is through email, internet, intranet, a company newsletter or by displaying details on a notice board, you should have the same access to this source of information as the hirer's employees.