I have been doing a little looking on Google and I believe the church is
Govan Old Parish Church.
This is a photo of the main windows:
This is a close up of the middle window and the same image superimposed over the alleged ghostly window set to 60% opacity:
With the superimposed image you can see that the alleged ghostly face matches pretty closely both the height and width of the angel. Also, the eye area seen in the OP closely corresponds to the area between the bottom of the banner held by the angel and what appears to be a fold in its robe.
Looking at the original close up for a while a vague face does start to appear around the area of the banner and the angels robe.
As some form of vague face is visible on a different image of the same window, I would put the alleged ghostly face down to a combination of a trick of the light and pareidolia.
This all took way longer than I expected due to stupid computer having a trantrum and why the pictures have my cursor floating about like a loon on them.
Edited by grendals_bane, 18 January 2012 - 10:04 PM.