Using my £5 android long lead endoscope I got some excellent video showing down the water channel from scuttle plate down to pavement!
Got some photos too.
Looks like I can see a seam not sealed near the top about 500-600mm from the car floor. Not sure exactly where it is yet but also found the drain channel hole through a horizontal brace totally blocked with crud and leaves. Once I managed to clear that with a hook on the endoscope I could push the camera right down to the pavement. Then I videoed. After that I put a hose down and flushed it out.
Here you can see through to the pavement!