Initially, we had to fill out a stack of paperwork. Luckily I had a list of my GCSEs and A-levels etc at hand, as this sort of information isn't something I would routinely take to an interview. While most of this paperwork was just standard info an employer needs to know, there were two question sections regarding previous sales or customer service experience, and what additional skills could be brought to the company, and a basic numeracy test with calculator provided.
Following this, we were given a sheet of paper with 4 example phone contracts and 15 minutes to prepare for a role play exercise in which we had to successfully make and close a contract sale to an assessor. We were allowed to listen to two example phone calls prior to this exercise, and it was quite easy. I wouldn't worry too much, provided you pay attention.
Finally, following successful passing of the role play, was a competency based interview. Four core competencies were explained by the assessor, each was given a real world example from the assessor's personal experience, followed by questions regarding hypothetical situations related to these competencies. Again, this was pretty simple - the answers are essentially covered in the explanation given before you give your own answer. Simply pay attention.
Overall, staff were very friendly. I received a phone call approximately 3 hours after the process to offer me an immediate start. Here's hoping accepting it was wise!