Where to start tackling water leakages?

Ok initially you need to try and stop water ingress. It may be something as simple as a blocked downpipe on your guttering, especially on a victorian property. So if you can get up there safely have a look and see if its full of detritus. Leaves etc are always an issue. This could cause your guttering to overflow especially in heavy rain as we are experiencing currently. Can you see water leaking from the gutters in the area that is the problem? Maybe the gutters are not sealed properly and have a small leak. You should also be looking at your roof, tiles of that age can easily delaminate and break down without you noticing, half a tile missing can cause the issue you have. These are the two first places to illiminate without spending a fortune. Is there a chimney near the area? The flaunching could have failed or another issue at the top of the chimney could be causing ingress of water. As for the wet wall, initially i would get the heating on and hire a small dehumidifier ( make sure the room is sealed as best as you can.) this is the first step to reducing the water. You may be lucky and the plaster is ok depending on the amount of water and just a rub down and repaint will sort the ossue. but you may be unlucky and have Larthe and plaster walls, the timber larthes could swell and pop the plaster needing it to be replaced. Fingers crossed. Get on to your insurance company Regards Dan weston

It docent sound like the guttering to me did you say there was a window above the landing were the damp is it could be a broken tile or a flashing really need to look