I moved this from the suppliers list as I'd like to keep it to suppliers links as much as possible to make it easier to read.
To answer your question, I believe there are some companies who will refill other companies' cylinders but they are few and far between and I'm also not sure about the legality of them doing so.
As a rule, BOC, Air Products and Air liquide cylinders are only exchanged (not refilled) by their own agents.
When you go for a 'refill' at one of the main suppliers' agents they simply take your empty one and swap it for a full one.
The companies that do refill on site will (should) have all the necessary training and safety equipment.
Xan looks around furtively to see who is looking, then says.
Yeah but mine is an ex rental bottle, a big one, it got lost on their books long ago when someone forgot to take it back ( not me ), I would kind of like to keep it.
My address is NOT being given out now ...............
Then it doesn't belong to you. Lost or not, it still belongs to whoever initially supplied it.
Good luck getting it exchanged but I know the majors have tightened up on identification o0f their cylinders now.
We also have members on here who work for the majors.
This bottle is like me and golum with the ring, its mine I tell you, mine. They can wrench it from my cold dead fingers, till then its now hidden under the stairs.
Actually being serious I spoke to the bloke at air products on the south coast ( i'm dropping no one in the cack here ) all those years ago when I had it given to me and I went for a refill, I was told the contract couldn't be found on his filing system, all he said to me was " it's only one bottle, I am sure we won't miss it, they go missing all the time round here, it isnt like I don't have a yard full ". I did get on well with him at the time, so who knows.
Having a 'spare' (stolen) bottle is not much use unless you also have an account you can use to switch the bottle through, with the supplier the bottle originally came from. That's the way the trick works. I can't imagine just trying to get it filled will get you very far nowadays.
The other thing to bear in mind is that cylinders normally have a test date stamped into them, if it's past the test date then I believe it's possibly only fit for scrap.