My experience with the NG lads as always been positive. Normally by the time I've got there they've always identified the leak and capped the meter, and left word with my customer of the general area of the leak.
I got called to a newly installed meter a few years ago which was fitted and left capped off due to a failed TT. Did my own TT when got there and couldn't even establish stab', soon as I shut the ECV the level in my manometer dropped to zero, asked the old dear and she had never smelt gas, asked if her gas bills were expensive and she said 'very'.
Long story short, she had an unused gas fire sat in her dining room and the pipework behind had rotted through in two places leaving two holes almost the width of the 10mm pipe. No smell of gas as it was being dragged out by the chimney flue.