Is it legal to store industrial sized propane cylinders in an empty domestic property?

I live in a cul de sac and opposite us is an empty property under development by a cowboy landlord. he has a huge industrial propane gas cylinder sat in the middle of his living room, curtains open for the world to see it. If vandals broke in amd detonated it, or turned it on and a stray spark detonated it, it would damage every house in the surrounding area. Is this legal and if not who in the uk would I need to report it to? thanks it worries me.