Hello I am a solicitor with over 15 years experience I will try to help you with this.
I can't think of any reason why you would want to get your insurers involved for something that sounds quite minor. Do you know how much it will cost to repair the damage and is this amount bigger than your insurance excess? Unless there is some rule requiring you td do it through insurace (which is unlikely) then you can take it up directly with the gas man or his employer.
I don't quite understand the second part of your question and would be grateful if you could explain it a bit more to me.
OK. well why don't you just pay for one then make it clear why you are only paying for one and see if anything happens?
I would suggest that you make a very clear written note of what has happened and sign it and date it. If there is anyone who knows about this and can be shown the bottle maybe they can help. But ultimately if you write to the gas man and pay only for one cannister and explain why then he will have to prove you are wrong to get payment for the other one. You may need to find another gas supplier though.