From your posting it's unclear why the gas supply must be isolated? If it's to prevent possible leakage and build up of an explosive atmosphere, for example while ignition sources are used as part of the asbestos stripping task, then breaking into the pipework isn't normally a sensible control, as that could introduce new leaks rather than prevent them. A more typical precaution would be to use a suitable gas detector to check the live pipework for small leaks, isolate where you wish and if that isolation is in the work area perhaps also do a general atmosphere test before work starts after every break. However if you need to break into the pipework downstream of the isolation as part of the task, then you also need to secure the valve in the closed position (so it can't be accidentally opened) and also to check there is no leak past it when you first break into the pipe. Capping the open pipe downstream of the closed valve might then be a sensible precaution - though you also have to think about what would happen if the valve DID leak and someone then removed the cap without any indication of the pressure behind it! (In a larger scale pressure system, you would normally ensure there was a bleed point upstream of where the pipe was capped or blanked).