You came to the right place
SlaverT-- You've come to the right place my friend. The episode features Mrs Peel, however it is not she who is tickled. A blonde enemy agent is tickled by Steed in what the majority of this Forum feels is the greatest mainstream clip of all time (as far as scripted goes, not Howard Stern).
To break it down for you, the hot blonde is part of a network of lethal secrateries. Steed hires her to do his bookkeeping (obviously a ruse as Steed is hip to her guise). At some point close to the end the blonde knocks on Steed's door. She is accompanied by a brunette. They attack Steed, who sprays the brunette with seltzer and traps her under a chair, then he puts the blonde over his knee for what I timed as 13 seconds of solid underarm tickling.
As far as genuine laughter, it looks real.
As far as time duration it's up there with any of the longest clips. I've seen other clips just as long but not as real looking. The only other scene I personally put on it's level is the one from Cement Garden.