Well it wasn't leaking on pickup of the car was it?? The roadworthy would clearly state that it was a problem, and that problem was rectified, so really after you exchanged money and papers the car was his and so is that problem. They'll try anything on sometimes, but really it isn't your responsibility!! After all you don't know how he drove it after he got around the corner, he may have flogged the ring out of it springing a leak nearby!! I wouldn't be too worried, I'm sure he'll go away eventually....maybe!!
Hope that helps.
Peter