Does anyone know how it is possible for my opponent (Equaliser769) to be at 115 minutes remaining for his next move, and 3 hours later have it show as 24 hours, without any move?

If he went on vacation briefly, it removes 24 hours from his vacation time (as a minimum) and adds that to his remaining time. I would assume that's what happened.

EDIT: Since he's a premium member, I assume he triggered the auto-time out protection, which put him on vacation. He has since logged in and come off vacation, meaning that the remaining time from the 24 hours gets added to his clocks.

Your answer makes sense, but now he has an additional 24 hours, without being put into vacation or having enough remaining vacation to get another 24. I usually don't root for people to time-out, but this guy has stalled out more than 1 tournament I have played in.