You are correct that removing the snap ring ( way down in the gas tube) allows the washer/ valve spring/ valve to drop out. The problem I had was that the valve seat itself was damaged.
These rifles were not meant to be dismantled further. Whereas in a modern gun the brass valve body would be held in with a screw, these early Benjamins had a soldered in valve body.To make maters worse, after soldering in the valve block, a brass transfer port/tube is soldered in to the valve from above with about 1/8th" prodruding out of the gas tube. The barrel is then lowered onto the gas tube so that the protruding transfer port enters the breech. the two barrel bands/clamps are then spot welded to the gas tube.
In order to do what I did I had to grind out the spot welds on the barrel clamps with a dremel tool in order to lift the barrel and remove the transfer port before de-soldering the brass valve block to remove it from the gas tube.
Most folk would have either put up with the slow gas leak or junked the rifle. We Brits are stubborn you know and I wasn`t going to let a few lumps of metal defeat me.If a human being built it,then a human being can dismantle it.
If I can figure out how to put the photos on here I will post before and after pics.
Anyway, I love this little Benji . It is so accurate and just sweet to shoot. As I am a 5`8", 10 stone weakling ,we were meant to be together.