Leaking Window inside below sanstone plinth when raining.

Hello all.. When it rains hard my living room window leaks within minutes. It drips from the plaster at the top where my blinds are fixed. I have checked outside both when it is raining hard and when it is dry and I cannot see any breaks in the silicone or the brickwork or pointing. It is a large gabled end victorian terrace house.. Has anyone ideas of where I can check to find this leak and repair it. Someone said it could be a perished plinth but how do I check that ?
I have added 2 photos.. inside and outside..
Many thanks... Phil

Is there a window above that one?

Looks like that window has been fitted too far out; it should be recessed further into the opening so the lintel overhangs more. If you look at similar houses in your area with original windows you'll see what I mean. I imagine this is a common mistake for window fitters who are now used to ugly modern houses where everything is fitted basically flush with the wall surface.
As you have it, the lintel is perfectly shaped to direct all the rain immediately into the joint between the lintel and window. You can reseal the join, but it takes such a hammering in that position that it will probably need redoing every year.

Charliedenson,
no window above just brick straight to the roof.

Gerrydelasel,
All the other windows on the house are set in the same way and no leaks from them.. Thanks for the advise and I will re seal the window just in case its a hairline and I cant see it..