1) PRESENTATION - you get the topic to present on a week before your ac, just make sure your presentation talks about BT, highlighting what they're competitors are doing and what BT is doing to beat them (This stage is meant to show how much you KNOW about BT!!).
2) TECHNICAL INTERVIEW - I found this stage quite less formal, its more like an hour long chat with your potential manager, but make SURE you brush up on your network terminologies like TCP, UDP, QoS etc. and also PREPARE questions to ask your interviewer (shows you're interested in the job), and even more IMPORTANT, do not mumble over a question you don't know the answer to, and always ask for the answer from the interviewer, don't just let him/her move on to the next question!
3) GROUP CHAT - I really enjoyed this stage the most! the advice I can give is that you should try to make as many friends as possible during the day. And also during the group chat assessment, DO NOT, i repeat, DO NOT argue! once you argue, then that's it, you might as well pack your bags and go home then!.....they look for leadership traits, and leadership is not ordering the group around, BUT bringing in people to the conversation, and SHOWING that you value everyone's contribution YET putting your points across!
4) CASE STUDY - You will be given 40 mins to read through a lot of information about a BT's product and prepare a SWOT analysis. Then you need to present this SWOT analysis to an assessor, who will ask you questions based on it. This wasn't easy as there was so much information to read through. Time management is key here.