I did an assessment day 18 months ago, the process is a very good way to get employed and quickly.
As long as you are confident and prepared on the day you should be successful.
I had a job within a week and after a year I left (I was maternity cover) they again helped me to find my current role which was another excellent opportunity in less than a week.
I highly recommend the process to grads.
Half of the top 10 aren't even RG...