Got to agree......Name and shame, if he will not give you your money back!!
He was really "clever" by not admitting it was a DOT bottle, and you were a easy target.
You can use it in the U.K. Just have to fill it yourself from a dive tank. You can get the dive tank filled from H-PAC, so you can use it at U.K. tourneys, just a arse about to carry the dive tank to the event.
The tourney organiser also will not be happy, cause it might invalidate his insurance, so you should check this out before you try and play with it.
Tom Allen was doing something clever with DOT bottles, give him a shout. Might be able to help you out.