In the very first episode, the character of Adam Klaus was played by Anthony Head. Although intended to be in every episode, the character was never seen in the rest of the first series. For the second series and beyond, Stuart Milligan played the role due to Head's involvement in Buffy the Vampire Slayer (1996).
The windmill that Jonathan Creek lives in, is open to the public. It's in the town of Shipley, West Sussex, around 6 or 7 miles South of Horsham, England. As of 2017-this windmill is no longer open to the public and unfortunately the owner is letting it fall into disrepair.
The car Maddie Magellan drives in the series is a Volvo 240 GL Estate which had an automatic gearbox. The gear selector can be easily seen in the episode where Jonathan drives the Volvo for the first and only time.