Crack near bay window

[JBR]

I'd be grateful for any advice about the possible cause of this crack near the bedroom ceiling, which is becoming wider.
It is above a bay window, so I suspect possible movement in the ground outside. However, there are no visible cracks in the wall outside. I'd value some advice before we pay for a structural engineer. Thank you.

 

[tell80]

The cracks are near the Bressumer beam or start at the beam. Check the level of the beam with a 1200mm spirit level.
Any previous history of damp penetration in that area?
Why not post pics of the bay roof abutment at the main house wall?
And pics of the external bay full height and at ground level?

 

[JBR]

The cracks are near the Bressumer beam or start at the beam. Check the level of the beam with a 1200mm spirit level.
Any previous history of damp penetration in that area?
Why not post pics of the bay roof abutment at the main house wall?
And pics of the external bay full height and at ground level?

Thanks for your advice. We did have the window frames replaced recently as they were rotten, so it could well be a wood beam rotting and failing to support the wall above. Actually, it's a bungalow, so no rooms above, but there is a gable end in brick at loft level. We'll get in a structural engineer to advise and, if necessary, have the beam replaced.
Sounds like it could be expensive!