Folly Tower and woodland. The New Avengers. Episode:Dead men are dangerous 1977

THE NEW AVENGERS: Dead Men are Dangerous (Sidney Hayers: Spring 1977)
Steed (Patrick Macnee) and Gambit (Gareth Hunt) attempt to enter the tower where Purdy is being held.
(Identified by Gareth Hughes, April 2003). http://avengerland.theavengers.tv/about.htm

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A view of the towers at begining of the sequence
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Steed (Patrick Macnee) and Gambit (Gareth Hunt) on their way to the tower. The fence is now covered with shrubbery. (Note: this view is along the circular path with the fence on the left belonging to the Thames Water reservoir)
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Steed walks towards the entrance of the tower.(Note: The door has many panels - so it is not the current metal door)
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Purdey (Joanna Lumley; her arm) in the tower. Of course this is probably studio or elsewhere. The steel pillars are not in the tower.
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One of the villains keeps watch
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Gambit jumps into the tower through one of the windows. Studio of course. The production team were clever enough to recreate the right type of window.