Advice needed on ceiling water damage - pics attached

[DavidFox]

I got water leakage which damaged ceiling. Asking a friend to cut off a block of ceiling has revealed inside. Here are the pictures: http://www.diynot.com/network/DavidFox/albums/

Clearly wet can be seen at the end of the joist. What could be the cause? Could the rainwater penetrate through the outside wall (as shown in the picture) and run along the joist? Or it is leakage from the roof? For example, the water runs along the band joist (which can't be seen, as a picture I got online and attached here)?

Just my guess (no experience on roof, leakage, etc.) and your advices appreciated.

thanks,
David

 

[Alastairreid]

Have you had new upvc facings installed recently?

 

[DavidFox]

Have you had new upvc facings installed recently?

Thanks. You meant the board that runs along the facing wall (outside)? If yes, it's installed many years ago, perhaps more than 10 years at least.