Can someone advise lead flashing

[misterboumsong]

Please can someone take a look at this picture of lead flashing on my lean to roof my internal wall plaster is bubbling and wonder if the gap above the lead flashing should be sealed. Is it possible water is getting in at the gap above the flashing? And through to internal wall.

What product should be used to seal

 

[Alastairreid]

It should be sealed, is that a cavity wall and how is the wall finished at the top?

 

[misterboumsong]

I ll grab more pictures in the morning

 

[misterboumsong]

Added another couple of pics of my Lean to roof