Cistern dripping from spare cold water pipe "plug"

[JamesA]

The cistern on the toilet we've just had installed has an inlet either side for the cold water pipe so you can choose the side you want.
The spare side we're not using has some sort of plastic plug in it.
However its been leaking round the edges and dripping on the floor since it was installed.

Tightening it hasn't stopped it.
Is this usual? If so what needs doing?
Does the cistern need draining, drying and silicon sealing round the inlet?
Or is the cistern just screwed and need replacing?

 

[JohnD]

Check the water level. It is usually below the height of these openings.