How can i trace leak?

Possibly there is a gap between the top of the frame and the brick work. Silicone sealant applied outside could resolve this.

Dave
November 2006

If your roof is lined with felt then it could be a broken/cracked tile/slate letting water in and it runs down the felt and if the felt has not been fitted properly, ie extruded into the guttering then the water will drop into or onto the brick cavity and onto the window lintel and find its way out the easiest route via the window reveal.
Unfortunately you'll have to get a roofer to look. if it's bad and you have house insurance why not claim if you consider it storm damage. Or it could be bad pointing.

Bob
October 2006