Problem with central heating system, pressurised tank system leaking.

You need to replace that relief valve or have someone do it real soon... its on the verge of failing. And it is important to replace it with the correct one... that is rated at the same pressure as the old one was. It's there as a safety valve in case something causes the system pressure to rise abouve a safe limit.

Where you would get one would be determined by where you live... and the make and model of that heating system.