Tell the gas board you can smell gas coming from his house.

They WILL gain entry....

He must leave heating on, to stop pipes freezing in winter. Can you surreptitiously access his outside stopcock and turn the water off? Sooner or later the boiler will run dry.....

Tell the taxman (or benefit fraud line) he must be taking in lodgers because you hear the wireless every day...

Actually, Bryn is right.