Dropped window reveal

[Lincoln10]

Hi guys,
I'm hoping someone can help me, I'm currently redecorating my mums house for her and after stripping back the wall paper in two rooms I've noticed that the window reveal (I think that's what it's called) had dropped and cracked.
Now I'm the first to admit if it's anything other than a bit of wall papering or painting I'm clueless so can someone tell me what it is made from and whether its a plasterer or a builder I need?

Cheers
Mat

 

[blup]

The part that's cracked, what is it made of, and how deep/thick?

The lintel above looks unaffected?

Blup

 

[Lincoln10]

Hi from what I can tell the front section is just a wooden covering if you will, I'm not sure what it's made of its around 2cms thick.

 

[catlad]

You may be able to get away with a can of expanding foam and a prop.

 

[Lincoln10]

Sorry I miss read blups reply the part that's dropping runs the length of the window, when I pull on it, it appears like there's nothing above it which led me to wonder if it was just old plaster board ?

 

[^woody^]

From that edge, it looks like a piece of fibreboard.

If there is a timber beam above it, then you can pop a few screws through in varoius places and fill over the crack.