Close coupled - cistern leaking underneath?

I've turned off the stopcock on the metal connector valve below big plastic nut under the cistern and felt around for exact location of leak.

It seems to be coming from under the plastic nut connecting to the ballcock pipe inside the cistern.

Is this easily remedied?

PS About a year ago, as a float, had round ballcock and arm replaced by little upside-down cup thingy (mentioned in case had some kind of bearing on it)

Thanks for any advice