Did they dissconnect one end of pipe and stick a gadget to take a reading? That is the air test.
I suspect the running of detectors is to identify where the pipes are concealed in the wall/ ceiling and floors.
The installation of brand new pipes should be viewed as alternative option due to LL reluctance on opening up to expose pipework and not viewed as a safer option.
If Towngas reported the 'small leak' and will subsequently be carrying the works then I would be suspicious as they have vested interest in the job. Again LL concern not yours.