Sounds like plaster walls and the ceiling is gypsum boards covered with just a skim coat of plaster. My house in the midwest US has a similar setup. It was during a transition period (40s-50s) from lathes to gypsum boards so the walls have lathe and then rough and finish plaster but the ceilings have gypsum boards covered with a thin finish plaster.
To hang pictures and stuff, I typically start the hole with a drill to get through the harder plaster layer and then tap the nail in the rest of the way. For heavy shelves in the closets, I've drilled holes and used those plastic inserts you tap in with a hammer and then put a screw into.
(Caveat: I'm a DIY type, not an expert.)
Diagonal cracks in old houses are usually the result of settling. Houses start out square, but they shift over time -- and turning a square into a parallelogram means stretching one diagonal and squashing the other. I've got some of these in my house, slightly older than yours -- most were patched and repainted before I bought the place but are still visible if you look carefully, a few were probably caused when I had a loadbearing wall replaced with a parallam beam.
If the cracks are new, and you haven't done anything like I did which might cause stresses to be redistributed, you should consider getting the place inspected by a contractor and/or engineer to make sure it isn't currently moving. You may need to install lally columns in the basement, or do some foundation work, to properly support the house so it stops moving.
Or it may be something more serious, but that's why you want the experts to look at it. (Neighbors had to demolish the old farmhouse they were hoping to rescue, when they discovered that it was both off the foundation AND had a broken main roof beam. Either would have been repairable by itself, but no reputable contractor would touch that combination.)
As far as repairing them: Once you're sure that the shifting which caused them isn't still in progress, it's like any other plaster repair. For small cracks, apply patching plaster, sand, repeat if necessary until the wall is smooth and level, and repaint. For larger cracks, you may need to use mesh tape to help support the patching plaster while it hardens. It may be desirable to undercut the crack slightly, to help ensure that the new plaster gets a good grip on the old.
If the house is going to continue to move, you're probably going to have to repair the crack repeatedly. Some folks suggest, in that case, using a paintable elastic caulk in place of the patching plaster; I'm skeptical.
Best Answer
Using a 2 1/4 hole saw, cut a plug from some 2x4 lumber. Then use some anchoring epoxy and glue the plug into the hole. Allow 24hr curing. Warm temperatures will speed epoxy curing.
Attach any wood screw to mount monitor bracket.
Heres a no muss/no fuss way to inject the epoxy. You may need to tape across the hole until the epoxy jells. They all mix in the nozzle and harden quite quickly. You use a caulking gun to dispense.