Hi All just seen this topic.
I have a 5inch Brittania with a ball valve regulator.
Due to troubles with my boiler, thought while it was off to change the valve, as it had been leaking by for its hydraulic test. (Its done well for some 20 years)
The surface of the ball had lost its finish.
So it was replaced and the engine back in steam.
A further problem with the boiler meant it was soon back off and inspecting the regulator the seals around the ball had melted.
Obviously not the same quality as the one before.
So it had failed due to the exessive heat in the smokebox.
So it looks like screwfix here I come!
Any suggestions on thread seal as I had the devil of a problem last time?