Hi Tone
I guess the oil is coming from the gear box there is an 'O' Ring behind the drive (not done the job myself)
BSA Part No: 65 - 3395 or Lyford Classic Tel: 01773 768900 their part no:LCS 2041AA price 50p etc I am sure C&D Autos will have it as well.
Some where I have read how to change it, if I can find it I will post it.
P.S I have a O ring but not the knowledge.
Regards
Nick