Slow Leak From Pipe in Airing Cupboard but not Sure What it is? Drainage and wastage systems and plumbing help, advice and answers

[Jeffaf]

We have a slow leak from the pipes in our airing cupboard. I can see where the water is coming from but I don't know what the object is. In the image attached you can see the top of a vertical pipe terminating in a chamber with a plastic screw cap on the top. The water is coming from under the cap. The vertical pipe branches from a horizontal pipe coming from part way up the hot water tank.

I'm assuming that the chamber and screw cap have some function other than to block the pipe. Should I just try to screw the cap down harder or is the leak a sign of something more serious going on in the system?

Thanks in advance!