overflow constantly dripping

Boilermate 2000. system approx. 18 years old.
Overflow is constantly dripping.
Have replaced ball valve as this was faulty.
Reduced the level of the tank.
However, after a few days, the overflow started dripping again. Turned off the feed to the tank and lowered level again. Eventually, the overflow starts to drip again due to tank filling up again. The temperature of the water in the tank is warm. I assume that it is being filled from the vent?
Any idea what is causing this? Note that a new bathroom radiator was recently fitted - system was partly drained and refilled to accommodate this.

When water in the cylinder is hot it expands by 4% this is probably why the overflow is leaking while the feed is turned off

When water in the cylinder is hot it expands by 4% this is probably why the overflow is leaking while the feed is turned off