I don't own a house with plaster so please take this advice with a grain of salt...
As you pointed out, the cracks usually appear due to settling. As a result, you ideally want your repair technique to be resilient against future movement. As far as I know, there are 2 main ways of doing this:
Method #1: For smaller cracks, fill them in with painter's caulk (latex caulk). Because it's a latex compound, it will (supposedly) expand and contract with the crack. It also makes cleanup a breeze.
Method #2: Use a joint compound specifically made for plaster work, tape over the crack, and apply additional swipes with larger blades to smooth the look. The technique is described here: http://www.askthebuilder.com/B38_Plaster_Repair_-_Cracks.shtml (the author suggests drywall compound but as Niall pointed out, it's better to use other compounds).
-M
CAUTION ALERT ! Dealing with a failing plaster and lathe ceiling can be very tricky. Before you make the decision to tear it down, which as Chris mentioned, a messy job, or simply put drywall over the existing plaster, you need to carefully evaluate the soundness of the lathing to the strapping or joists. What I mean by that is, are the lathes still firmly attached to the joists and the problem is the plaster "keys" failing from around the lathes, or are the lathes themselves falling away from the joists?
With that said, covering the old failing plaster with drywall is fine. If the lathes are solid to the joists, then putting drywall directly over the old and using screws long enough to reach the strapping or joists will work fine. However, if the lathing itself is failing, then you must install strapping 16 inches on center, perpendicular to the joists, securely screwed to or ring nailed to the joists. The purpose of strapping is twofold. First reason is to support the failing ceiling securely to the joists. The second reason is that if you just screw drywall to the failing plaster and lathe, the weight of the ceiling will be against the drywall and could pull the screws through the drywall board allowing it to sag or joints to crack now or in the future.
I must say that even though tearing down the old ceilings is a big job, it certainly gives you a clean start and an open view to correct any old wiring problems, add new lighting circuits, install new flush mounted electrical boxes, and install new level strapping etc.
Best Answer
Looks like pretty generic coving to me.
If you're not sure if it's a generic size... then find out! Easiest way to check that is to walk into your local DIY store (B&Q, Wickes, whatever) with that small bit and check to see if they stock something similar.