Hi, I can only go by my one, which was for welding/fabrication, so a different trade but am guessing it's probably a similar format?.
mine was very relaxed and lasted about an hour, it's done over Skype by someone who has worked in your industry. All he did with me was go through my references and asked questions regarding the type of work I'd done and how I went about doing it?. They just want to know that you are at the same level or above as someone who has just qualified/finished an apprenticeship in your trade.
try not to worry, as long as you know your job you'll be fine! Hope this has helped. Good luck.