Neighbourhood Watch by Alan Ayckbourn directed by Lindsay Jones

Brother and sister Martin and Hilda live together, leading a quiet existence in the comfortable confines of the Bluebell Hill development. As they are preparing for a housewarming gathering, their peace is shattered by a young trespasser in their garden.
They and their neighbours congregate and together agree to form a neighbourhood watch group to safeguard against further incidents. But after their beloved garden gnome is thrown through their window, matters swiftly escalate.
What had begun as a well-intentioned scheme for a safer community, ends in violence and acrimony, with blissful comedic moments on the way.