E30 318i: Water in rear footwell - windscreen cowl leaking? Koi carp anyone??

After heavy rain I noticed the rear passenger foot well in the E30 is full of water. Can't see any signs of water ingress through the door seals or any stains in the headlining, but am told a common cause of this is the windscreen-cowl, so will be looking into this soon. For now, I have drilled a nice drain hole so at least it won't be filling up again.