I've worked at JL for nearly two years (my last day is next sunday tho cos i'm off to uni hopefully) - the group assessments are really easy, purely to assess your skills in communicating and working as a team, and how confident you are to speak/put ideas forward! i did things like selling items to fictitious customers in a group, it's nothing too difficult so don't worry
dress like you would on the shop floor - black, white, grey, pale pink or pale blue are the JL colours, you need to look business-like, so trousers, blouse and blazer or a black dress and blazer would be perf! and for guys it's obviously a suit
time of day makes no difference tbh, but morning interviews means it's out of the way quickly i guess lol so that's better
if anyone has any other questions let me know!!