If the underground pipeline is indeed 50 years old it will be metallic and should be subject to a full survey to determine it's condition, which will not be easy to do. It would be better to consider replacing it with PE.
I came across a 50 year old town gas converted to LPG installation in a college a few months ago.
1" 1st stage gas emerging into a basement boiler house (boilers running on 35sec), into a 2nd stage regulator, no upso, vent not piped externally, which then was piped into existing 4" town gas pipework running 6 floors up a service duct to the canteen feeding 2 labs on the way. Not an ounce of fire stopping between floors, no accessible ECV's and, best of all, pipework supported by blue rope tied from the heating pipes! The BST was across the carpark with some 40m of pipe buried under tarmac!
Needless to say I had the installation marked as ID before I tightness tested it. And when I did TT it my pressure dropped faster than a footballers morals in a seedy nightclub!
We've now ripped the whole lot out, PE across the carpark. 2nd stage regulators with UPSO's outside where possible, and where not (quite a bit of the college campus is accessible to the general public besides the students!) we've only the minimum amount of 1st stage pipework inside the building with the vents piped to the open air.
We've also installed gas proving systems, gas detection systems and extract fan interlocks in the kitchen.
I'm due to go test, commission and purge the pipework next week.
The tightness tests involved may, depending on volume, take hour(s).
This ain't the case cocker! There's a maximum time limit allowed depending on the type of gauge you're using, e.g. Water with an sg of 1.0 has a maxmum test duration of 30 minutes, electronic with 2 decimal places, 15 minutes, and electronic with no decimal places, 1 hour. if your TTD exceeds the maximum permissable time for the gauge in use then you must section parts of the installation off and test them seperately!
easy innit!