Fuel leak at sender unit

[pawmaro]

Hi have noticed smell of petrol and crap consumption for some time found leak at tank sender unit. There is what appears to be a aluminium bung about 25mm dia with a allen key socket. The fuel is seeping around this does anyone know if this is the drain bung or for securing the sender to the tank? I have tried to teighten it but no joy it will loosen.
I am hoping its just a case of a new seal but filled up tank yesterday.

Your comments appreciated
paul