Leak from toilet cistern inlet pipe.

Hi all and thanks in advance for any advice that you are able to give me.

I am having an issue with the inlet pipe to my toilet cistern. I have attached a photo of the setup but the water is leaking from the join between the metal nut at the top and the plastic pipe coming from the cistern.

The leak can be stopped by isolating the water supply using the screw shown in the photo, but whenever it is opened up again (to fill the cistern) the leak starts again.

Does anybody have any advice on how best to fix this or what may be causing the leak?

If you are certain that it is there and not higher with water running down the thread above then you can try a gentle nip up on the joint or dismantle the joint and replace the fibre washer under the nut.