Brass central heating elbow joint leaking at connection to plastic pipe
The fitting looks like it’s a Sharkbite which is a type of push-on fitting that can also be removed by pushing the plastic ring at the end of the hub back into the hub by use of a special tool, an adjustable wrench, or even sometimes your fingers.



Before you remove the fitting, you will need to shutoff the heating system, relieve the pressure, and if possible isolate and drain the affected line.

Once that is done, you can remove the fitting and inspect the pipe ends and the fitting for any visible defects or damage. Here’s a instruction video on how to install and remove the fitting.

Note that there should be a sleeve that inserts into the end of the plastic pipe that looks like this:



It strengthens the end of the plastic preventing it from being squished by the clamping force of the fitting. If it’s missing, it might be the cause of the issue. More info here.

Once done, you can reinstall the fitting or replace it if needed. If the end of the plastic pipe is damaged, you may need to cut it back and add a new piece. Sharkbite also makes couplings as well.