Leaking Thermostat Housing.

In June I replaced the thermostat because it was seeping water on the join of the housing. Yesterday I noticed I was loosing a bit of coolant and on investigation the new thermostat is leaking in the same place. I got it from my local independant and although it had never been used before, the state of the box and the heavy covering of dust suggested it had been sitting on his shelf for quite a while.

Has anyone experienced this before? Is it a known weak point? Or has sitting around on a shelf for unknown years taken its toll?

The leak is right on the seam where the 2 halves join together. Just interested to see if anyone has had a similar experience.