(Columbia crime series, 1979-84)

Adventures of an unlikeable wealthy couple who travel the world solving crimes by getting in the way of the police.

Episode 1.04: Death in the Slow Lane:
Partly set in London, and stock shots are used to establish this - note the three AEC Routemasters:

……but street scenes are punctuated by a red 'London' bus passing multiple times. It's actually former Southern Vectis 547 (ODL7), a 1957 Bristol Lodekka LD6G with standard Eastern Coachworks body, by then residing in California:


Episode 5.01: Two Harts are Better Than One:
However, this episode IS filmed in London. The smug Harts are driven through London by their faithful manservant Max - reflected in the windscreen of the car is a London Transport Red Arrow Leyland National 2, while a DMS type Daimler Fleetline is seen behind. Almost inevitably a Routemaster is glimpsed:

(thanks Denise Annells)