Just for information.
I did get the rifle in question in hand and found that it leaks at a high rate, so much that after filling it to 200 bar, each consecutive shot lost about 30 fps and within 5 shots it went from 916 fps to less than 750 fps. After waiting a few minutes the next shot was less than 400 fps. There is no gauge on the rifle to see what the pressure is.
I did open the rifle and is was not a problem with any O-ring. I found that the valve itself was leaking, not any O-ring on the valve unit. I cannot see damage on the teflon ? valve seal (or whatever material it is) or the valve seat. It is difficult to see detail on the valve seat as it is, what looks like stainless steel, bushing pressed inside the bronze valve body. I don't know if it is leaking around the bushing / valve seat or at the seal itself.
I will contact our local FX dealer, Patriot and if they cannot help, I will contact FX Sweden. Since it is an old PCP I doubt if they will have spares.