A picture says more than a thousand words...

Identified quite some leak a few days ago. Don't know for how long this has been leaking, but I definitely did not have this leak until after a 1100 mile drive in Italy. I would say perhaps 100ml have dripped out since then. Managed to tighten the plug a bit, which appears to have stopped the leak (will know for sure tomorrow morning).

But what plug is this? The filler plug is on the side, no?