Octavia II Gearbox Drain / Fill Plug

Hey all,

Managed to get under my partners car at the weekend to service it and change the oil. Whilst there I was quite taken by how clean the engine was, but there was a patch of what looked like oil on the undertray. This was later discovered to be coming (I believe) from the gearbox drain plug. The car has had a new clutch, flywheel and replacement box in the last twelve months and so I can only assume there's an issue with the drain plug or a seal.

Looking really closely at the drain plug, I see no seal at all and yet a parts diagram for this box (lhd 2011 MY) it shows what appears to be a plug and a separate seal. Can anyone advise at all? I guess the gearbox is going to get yet more expensive oil and a change again but I'd like to know if that seal should be there if anyone can advise? Thank you so much.