This could be a helpful link:
http://www.rethink.org/living_with_m...ghts_and_laws/
Also, your hospital should provide information when you're sectioned, usually in a leaflet and by talking to you, and make sure you understand your rights. You should also have the opportunity to speak to a mental health advocate (though the staff may not directly offer this) of some sort who can talk to you about your rights and support you at tribunals and the like - it might be worth asking your named nurse about your options.