What a coincidence, I just joined the forum to ask this exact question and found it had just been asked -spooky!
When I last visited, I looked at fitting a light in our store, but, before doing so, I tested the wires at the light position with the switch turned off and on. Power showed on the tester both times. I then tried with our consumer unit turned off, power was again proved. I came to the conclusion that the lights must come off 'The Landlords' supply and was about to ask the forum who I could speak to on site. As you probably guessed I am not an electrician. Would the common always be live? - I don't fancy getting frazzled by leaving it to chance. Maybe our wiring is different to Pitby's - Any advice welcome.
Paul