Central Heating Drained and now Expansion Tank is Filling to Overflow

[DMStue]

Hi there,

2 days ago we had a valve on a radiator fail.

A plumber drained the system, replaced the valve and refilled the system.

When I got home that night I noticed that there was water dripping out of the overflow pipe from the loft. Which was new.

Checking in the loft, I have 2 tanks, a large one and a small one next to it. The small tank (which I believe to be the expansion tank?) was full to the overflow outlet. I manually drained some water and checked the ballcock mechanism and that is functioning fine. However the tank keeps filling up to the overflow.

Its not a major issue, but I would like to remedy it. Also, the annoying drip drip drip on my conservatory roof will eventually get my wifes attention and then it will be a much higher profile issue!.

Does anyone have any clue what the problem might be or how to fix it?