I've got a fireplace in my house which has never been used. Its connect to
the gas via a "gas restrictor elbow" as seen here
http://www.toolstation.com/images/li...bbig/85480.jpg

I would like to stop the gas going to the fireplace while i do a bit of work
around it. The top part of the elbow appears to be a safety cap which then
reveals another screw section. I thought it would be a case of just screwing
this down which would then block the line but it just kept moving down, I
did not want to screw it any further incase it fell through and then meant
there was a leak.

Is it just a case of screwing the middle down till its blocked?