Hi Nic,
The tribunal usually ask for everything to be submitted at least 2 weeks before the date of the hearing. They might accept something on the day, but they are not obliged to do so. It is an integral part of British law that both sides of any dispute see all the paperwork beforehand. There is also the fact that a strong submission might bring about an early decision and stop the case needing to go to tribunal.
You might find seciton 4 of this thread helpful
Appeal tribunal advice if you have any further questions, please come back and ask them on this thread.