I completed it at the end of January this year. Honestly just relax and you'll be fine. You start the interview and there are 3 pre-recorded video clips with Openreach staff asking you questions. I think you get a minute to prepare an answer after the clip ends and then if you don't like your first answer you can have a second go. My questions were (as far as I can remember)
1. Tell us about yourself
2. What do you know about Openreach
3. Why do you want to work for Openreach
There's also a lot of people saying shirt and tie as if you were going to an interview. I wore a plain black t-shirt and got through just fine. A shirt and tie will definitely make you look smarter but it's not needed. Just look in a mirror make sure there's no food stuck in your beard and neaten yourself up!
Good luck, take it easy, you'll be fine.