you will just have to pull out the plasterboard and have a look see. you won't find out otherwise. but it sounds like a tiny water leak, which may have been there for a long time in the flat above and the stud wall has cracked the ceiling enough to let it through. some tiny leaks can simply evaporate in time and not show, but in this case it can flow downwards if the stud wall has created the opportunity to do so.
Ok, admittedly posting a few months later. But just in case anyone searches for a similar problem and comes across this thread - turns out that KIAB was exactly right, and it was a slightly warm heating pipe cutting through the wall. We turned all the radiators off in the flat (no issues because it's summer) and the ticking stopped. So the lesson learned is that a warm pipe cutting through a stud wall where the insulation is quite tight to the pipe can cause the stud work to make noises at random places across the wall, even if the temp feels fairly constant, and isn't coming on and off intermittently.
Yes we know bloomin obvious really, first thing most of us would have suggested ....but!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I seem to remember it was about the "Enfield Poltergeist". I have read and still have the original book "This house is haunted" by Guy Lyon Playfair. Frightened me to death at the time.