Had Window install caused damage to wall - Pic Included

[Hanuki]

Had double glazing installed, quiet a while ago more then 2 years know stripped my wall paper to this surprise- Can someone please suggestion what i can do to repair this?
What material needed?

I am completely clueless I like a rock slate : /


I did direct links to the pictures using tiny pic i hope there no issues

http://i46.tinypic.com/2heizgo.jpg
http://i50.tinypic.com/2dhd6k3.jpg
http://i48.tinypic.com/2pqrs4l.jpg

* Added - It seems like it has collapsed cos i can put my fingers under the dry wall where the window seal is?

 

[Harbourwoodwork]

Itlook's like very old render that has perished ,it could be old victorian lime based render all the loose stuff need's hacking off ,probably in the area of the window ,caused by years of condensation and leaky windows its wants repastering with sand and cement render and plaster finish ,depending on how much comes of,it could be a job for a spread,that also depends on your plastering skill level

 

[screamer]

as said in the above post, hack off all the crap old plaster, re-coat with a renovating plaster

 

[HERTS P&D]

Hack off and re-plaster.

This is not a DIY job.

Andy