I went on one with HSBC in January (got the job btw)
there was 10 of us and they split us up into 2 groups. First exercise we had to ask 2 questions and figure out which card was missing. 2nd we had to use a box and draw and stick stuff on this box to show what HSBC means to us, they also made us work out what materials we were going to buy beforehand and we coudl only use those materials whilst making it.
I had a numerical reasoning test and a verbal reasoning test but this wasnt at the assesment centre was a different day.
Numerical stuff is easy if your good at maths, mine involved number sequences and what was the missing number. Hard aprt was that i had 30 seconds to answer each question
my verbal reasoning was to read a paragraph and answer questions on it, had a few paragraphs to read.
Just be yourself really, I was. Had a good crack on with the assesors I spoke clearer than I normally do so they could understand my accent