I think I will now stick with my suggestion and purchase, in post #4. That is a canister of reasonably priced argon gas, which is working fine for me, when topping up bottles of chemicals.
I have just double checked many sites, on which they quote: 'Argon is completely nonflammable.'
The same cannot be said of any of the other gases suggested for use.
I'm particularly careful about what I want to use, as my darkroom is in a solid wood cabin at the end of the garden.
Terry S