Thanks for your comments. Part of this weekend has been spent in the garage. It took some time but now it's done (almost...).
First of all, the leak was due to the plug behind the flywheel.
I took the engine out. Not too difficult as I was able to borrow a patient lift (former medical device) which now treated a new patient, my X/flow engine. Only the rear one was leaking but I replaced all four. The remaining three looked OK from outside but looked quite corroded on the inside. Most likely a matter of time for the other to start leaking.
I also replaced the front and rear crankshaft oil seals as the engine was leaking some oil. Not very much but I think the seals are the original (from 1973) so it's probably a good idea to have them replaced.
As a bonus I found the clutch disc to be a bit worn so I will replace it as well.
