Ok so I survived the day.
Got there early, a few people also waiting, apparently around 100 were scheduled to appear during the day for one of the three presentations (I went for the middle session at noon).
Sat through the presentation, there wasn't much I didn't already know there about the company, but afterwards went to see Hayley the talent manager (as per the letter), and she asked me to just hang around until everyone else had gone. The others filled in application forms there and then, which will be sorted through later that day, or monday/tuesday, then the successful ones will be called for interviews.
After everyone else had gone, I was called for interview, which was in a small side room with two guys, which lasted about 35 minutes, roughly what I expected. The usual questions... what do you know about the company, can you tell us about your experience in (this) which is part of the job requirement, what would you do in this hypothetical scenario etc - nothing really too taxing.
At the very end, they asked to check that they had an up to date contact number for me - which may or may not have been a standard question, but I thought it was encouraging. As far as I can tell, in that middle session, I was the only one that got an interview.
So... all I have to do now is wait. I don't know about the other two sessions, but there were certainly a very mixed bunch there, some in suits, others in jeans with their arse hanging out. I guess whether or not I get called forward for the next stage depends very much how strong the other candidates were, and where I fall within that range. All I know is that I did the best I could on the day, and that's what really matters. If it's not good enough for them... then I guess it's their loss.