Well the stat turned up at long last - only 5 days late
Just fitted it and was going to test my old one but there was no point, the only desaster I had was that stupid plastic plug.
The top of it snapped off just above the thread but I was able to remove the threaded part.
Luck for me I had a steel plug that fitted, if anyone has the part number for the plastic plug let me know as the steel one will react.
Now back to the stat - this is what I found.
About 1/4 of the seal had gone - hope you can see it in the picture.