Another thought.... have you had a good look in the roofspace? In older houses, it's often not made up to full height between the houses, and someone (perhaps a resourceful and agile younger person) could crawl through, taking a rope ladder to climb down into the other house. Wear latex gloves, opportunities for devilment or just alter the timers and turn the volume down. Take a portable battery powered vacuum cleaner to eliminate any mess. Clean up behind like the last Red Indian in the file.