Sorry, what I meant was that the "test point" which you have circled on the photo isn't a screw. I've tried turning it but it it doesn't screw in or out...
The surface of the test point is flat so I had to use a small pair of pliers to try and tighten it rather than being able to use a screw...
Thank you so much for the responses.
The test point on mine is where the leak is coming from. It doesn't have a screw marking on it, it is exactly the same as the picture you have posted.
I'm hoping someone might be able to help.
The double check valve under my kitchen sink is leaking. I originally thought it was the bottom join which connects to the pipe that was leaking but it is the middle (which doesn't have a screw marking on it) - which I believe is the valve itself...