Well after consulting others the consensus of opinion is that it definitely wasn’t filmed at Radlett/Frogmore as the landscape / tree line is completely different regardless of which runway was used. Bovingdon has been rejected due to long grass, a missing aerial and a runway that appears to be disused when Bovingdon was still open in 1967.
The strange thing about Bovingdon is that the natural features of the landscape all point towards Bovingdon as the woods and tree line on the horizon all line up.
So it must have been filmed somewhere else but where is a total mystery.
Bovingdon was used for a short sequence in one episode where No 6 escaped and was taken up in an aircraft ( Hunter or was it a Meteor?) to try to find the Village
No that was filmed at Chalgrove, Oxfordshire, the home of Martin Baker