As mentioned above I would try and get an experienced gas engineer who is happy to spend a bit of time testing each section of pipe. While not impossible it is unlikely leak is on the Tracpipe section if it is one long run. I would concentrate on meter connections, boiler connection and any valves in the pipework( especially in the apartment if that is where the smell first appeared). For a good engineer it won't take long to determine exactly which section is at fault.
Why not post your location and see if any of the lads here are able to help?