Not safe for 12 month's and then unsafe no!
Twelve Month's is a reasonable amount of time for a dormant service to remain in place with a temporary cap. The plug cock and cap you have is, or was, more than likely still safe but it is not considered a permanent disconnection.
I think you are missing the point. They own the service pipe as I have said earlier.
They are constantly upgrading, altering and maintaining things for all sorts of reasons. There are more regulations and standards than you can shake a stick at.
Read this for example - this document should be
read in conjunction with this one - decommissioning unused service pipes is something that is done for health and safety reasons.
I personally think the message not getting through to the right department is your issue. You perhaps need to concentrate on getting to the bottom of that first and foremost but I don't think you will get any joy.
If you take them to court, I expect it to cost you considerably and get you to the stage where you will still need to apply for reinstatement of service.
The two documents I posted links to are among a myriad of them. The Gas distribution network companies, do know their legal standing and duties.