Cannon and Ball: Series 8 (1987)

Tommy and Bobby need to find a new flat. They are show round Paradise Mansions in Soho, then look round a flat in Park Lane. The Park Lane flat has a security lock on the door so you need the key to open it. The estate agent pops out, and the door locks behind him. Tommy and Bobby are trapped in the flat. Eventually they escape and decide to buy the flat. While waiting for the guests at their housewarming party to arrive they realise they have lost the key again.