Initial phone call with company is straight forward, 'What can you tell me about the company', 'What has led you to seek a career in trading?'. Pretty simple stuff, that anyone with a bit of conviction and research can breeze through. You will get asked around 3-4 mental maths questions, 'What is 4 to the power of 6?', 2-digit multiplications and 3-digit additions, again relatively easy but in the moment you can easily bottle it.
Assesment center was more of the same, however despite the simplicity of the tasks given to you, they need to be completed perfectly. 'Good' won't cut it. Written maths was 80% 'find the next number in the sequence' type questions, relatively easy if you practice and know how these problems work. The next 20% was real world maths, i.e. 'Two people leave for a destination at the same time, going different speeds, and one stops during the trip. How many minutes apart will their arrivals be?'; 'Company X has sold 20% of its t-shirts at 30% off, 10% at 40% off and the rest at 15% off. The price of the t-shirt is $7.95, and in total they sold was 2000, how much did they make?'. Again, relatively easy but the standard is high so getting one or two wrong will end in you being cut.
Concentration grids can be practiced, I would consistently complete them under 4:30 at home on my own computer, but my average time was over 5:30 at the AC, so expect your completion time to increase. Practice with distractions, i.e. music you're not familiar with, someone looking over you, having a conversation with someone else, and all at the same time just to be sure.
I got cut after these two parts. I didn't perform poorly, but the standard is high and the people you'll be with during the AC are very smart. Most of the other posts on here describe the experience accurately, except for the ones saying the interviewers were unpleasant, which isn't true, they're just straightfoward and you shouldn't expect any less.