Jules wrote:

Theres a ledge on the outside of the window frame, about 3 inches
wide which runs the length of the outside window frame (see the
photos). Its just above where the water gets €“ is the most likely
suspect?

I'd certainly start with that; as a very cheap test, seal along the join
with some sealant and see if the problem goes away (nowt wrong with it as
a more permanent solution either, but you'd be better off stripping all
that flaking paint off, re-painting and then sealing afterwards if so)

I've a rather similar problem.
What sealant would you recommend?

--
Timothy Murphy
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland