It's Sunday these days 2_deck, though as you say arrivals are staggered throughout the afternoon and the AFCO will sort out transport to fit in with the pick up times at Raleigh. Generally it seems to be late afternoon, but as I say, depends on the arrangments made between Raleigh and AFCO.
You will need to arrive at Plymouth train station between 1600 - 1800 on the Sunday. Your AFCO will advice you on the train times and you will be met at the train station by a member of the training staff. If you are arriving later than this they will be aware of it and they will wait for you.