duplicate bridge revoke not agreed upon
There is a standard pattern here that may be applied to numerous difficult decisions:

Politely inform Declarer that as he cannot contest the explanation of play provided by the defenders, you must tentatively accept their version, and thus are required by law to assign the normal revoke penalty. Inform Declarer that he is entitled to appeal this ruling, and that you will promptly assemble a suitable committee upon conclusion of play if he so requests. Ask declarer if he wishes to make such an appeal, and if he replies in the affirmative arrange for the three best and most respected players of the club to act as the committee. 

Allow declarer a few minutes after the game to refresh his memory of the play with a (supervised) examination [of the board], to prepare a rebuttal to defenders' description of the play.

In order to ensure that the committee proceeds efficiently remind them that their prerogative is strictly to determining the facts, and that given a determination of the facts you will apply the rules by the book. You are the Judge, and they are the jury - not the other way around. Declarer presents his case first, followed by the Defenders, followed by a simple recap by Declarer. Committee members may then pose questions to both sides. Get a determination of the facts from the committee, and apply the Laws; politely explain the result to both parties. 

Done correctly the entire process (for a simple determination of facts such as here) should be over in less than 10 minutes. Don't let either side or the jury waste time.

Remember to thank both side later for creating a situation that enabled you to practice your committee work.

Update:

The key here is that "I don't remember." is not a defence in any circumstances; but "I need time to refresh my memory in order to prepare a defence." is a valid request.