Worcester heatslave 20/25 leak from valve?

Hello! A complete novice when it comes to boilers, so bare with me! I've got a Worcester heatslave 20/25 and turned the heating on for the first time this winter today. Started to have a leak, examined it a bit more and found it has leaking from gold coloured metal bit at the top of water tank. The top of this gold bit has a small black plastic lid to it, which was loose and the water was coming out of this. I tightened it and the leak stopped, but I just wanted to check that's ok to do and it's not supposed to be loose for pressure reasons? Any advice would be great!

Thanks!