Leak below a flat roof

[Bez]

We have a leak that comes inside our bedroom window.
We have tried a few things but are running out of ideas now - can anyone help?

We have had the flat roof of the bay window done.
We have had the sash windows serviced and draught-proofed.
We have had the masonry repointed.

Still, the leak occurs. It is very localised to just one part of the bay window and comes through on the inside of the window frame below the area where the water goes into the pipe from the flat roof.

Any ideas what could be causing this?

[buildersmate]

......."goes into the pipe from the flat roof". Not sure what you mean. Have you got a conventional guttering where the water drips off the edge of the flat into the guttering, or some other arrangement, or are you referring to a leak much further down the window frame - behind the downpipe from the gutter?

[Bez]

There is a small opening at the corner of the flat roof from which a pipe leads to the main downpipe.
As the flat roof is surrounded by a parapet I cannot see the actual pipe until it comes out the other side.

The leak is coming through right at the top of the window frame, although there is also staining on the wood either side of the window and also the plaster (beneath the now "removed" wallpaper) is damp underneath the window.