How severe is the leak? All of them leak. The seal on the back of the crank is an acme thread. When the engine is rotating the thread flows the oil back into the crankcase. When not moving the oil will seep. Keep in mind Bentley engines where designed long before anyone cared about oil leaks. As W.O. Bentley would say "normal seepage". Feel free to contact me if you have other questions, or send me a picture so I can see how much oil you are talking about.

Consider buying a 'technical facts' book from the Bentley Drivers Club. The leak is probably coming from the crankshaft rear main bearing. I have never had the chance to work on W O Bentleys, only later Bentleys which were built by Rolls Royce but it's the same on most engines.