had similar a few wks ago did a gas cert and found major drop - removed boards and found nailed pipe -repaired that and and the 1st piece of pipe and meter union off the meter . I did a tightness test an all ok . got a call 3 wks later off landlord saying customer smels gas and called Transco who capped meter and replaced regulator and flexi due to leak . that engineer slagged me off to the customer . it was tight when I left it -apparently engineer said there was a small pin hole in the flexi stainless pipe . I assume the pipe was knocked when the tenant put stuff back in cupboard and disturbed the flexi - like an mot on a car its only falied at the time it was done