Seeping pipe connection to Loft Cold water plumbing advice

Seeping pipe connection to Loft Cold water Had a problem with hammering noise coming from pipes (triggered by hot water, though it could be eased if you put the cold water on).
Went through everything i could, checking joists etc. Problem appeared to be the ballcock in the loft cold water tank.

Ive replaced it but for the life of me i cant stop seepage out of the nut for the main connection. Though ive only tried
1. Going tighter, getting to the point i'm not sure i can get it any tighter!
2. adding the pfte tape to close any gap

Still getting seeping from the nut, left a tea towel wrapped around it overnight and its soaked this morning.

Im a real newbie and feel the answer is obvious (more tape??) but seems the majority of the job is done, i really dont want to bring someone in but im all out of ideas and know my limits, which are pretty low it has to be said.

For info - I wrapped the tape around the brass thread of the ball cock, no tape used on the cold main thred/nut. See photo

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