It may look like a lot of motorcycles in this episode, but there are in fact only five, or maybe six. Mostly BSAs, plus Cathy's famous Triumph.
The motorcycle riders were members of the Riverhill Riders Club, who worked with Surbiton Police Station for charity events.
Two lines are cut from the script. Cathy says "I have to go out now." Two of the gang barge past and she leaves, but in the original script, to heighten Jessy's fears about Cathy and Dave, Jessy asks, "With Dave?" and Cathy replies, "Yes, with Dave."