Steed Biography
John Steed -
Stuntmen  and Stand-ins
...information from the episodes
They're often not even in the credits of The Avengers, but these men were important: Patrick Macnee's stuntmen.
While it looks like that Patrick Macnee did his fights in series 2 and 3 quite often without stuntmen, they did the stuntwork for the actor frequently from series 4 on and also drove the Bentley in many of the scenes.

Four different people worked as Patrick Macnee's doubles/stuntmen: Ray Austin (stunt coordinator/arrangment), Joe Dunne, Rocky Taylor (series 4 and 5) und Paul Weston (series 5 and 6)
Stand ins for Patrick Macnee: John Wilder (such as in the Cybernauts), Pat Ryan (series 6), Paul Weston (series 6).