Last time I had just the one assessment centre with a whole day of exercises. Thinking back to it now, I was uber nervous all day!! But I powered through. I have no idea how it's being done this year. I'm trying to be in semi-regular contact with Ami Beeton and the NGDP via Twitter. But communication seems a tad informal. I did my assessment centre in Leeds, although they had a section in London.
Also on a side note, I have received confirmation via Twitter that there will be information days coming soon!