Coming back to this, Have never had a valve fail on a pot. Suspect I have a pot that is so old that doesn't have a safety valve.
I can't see that there is a problem as if you have the right grind and don't tamp the grinds, the screen is clear, the seal is good and the tube is clear then how is there an issue. Stuck a bolt in and stuck the pot on the stove. The valve has failed so openes and doesn't build enough pressure to force the water past the weighed top to make the creama.
Now to get a new valve, or might try an find a new o ring, as finding a new valve in the UK might be difficult and or very expensive. Postage being more than the valve.
Also there is no way to know if a valve has failed to closed rather than open....