Tony's plan left a lot to chance (SPOILERS)
SPOILERS - SPOILERS - SPOILERS
I guess that should be enough warning for everyone....
Superficially, Tony's plot to have Margot murdered seems ingenious and flawlessly planned. Even his last-minute improvisations, when things go wrong, are actually very well thought-out considering he had little time to do so. Yet for all that he left a lot to chance in his original plan. Its success depended on a lot of things over which he had absolutely no control just happening to fall into place at just the right time, and if any one of these assumptions didn't pan out it would have wrecked the plot then and there. Consider:
>>Tony phones Swann (Lesgate) at his home Friday evening to ask him to come over that night to discuss the car he had to sell -- actually, to be blackmailed into committing the murder. But what if Swann hadn't been at home when Tony called? After all, it was a Friday night, when many people go out. What if he were at home but simply couldn't make it that night? Tony's plan hinged on having the murder committed the very next night. Letting this crucial element go until almost the last minute is crazy.
>>Tony tells Swann to put the key back under the stair carpet as he leaves. Of course, the critical plot element is that he puts it back under the carpet before entering, leading to the mix-up in keys which eventually trips Tony up. But why not tell Swann in the first place to return the key back under the stair carpet before entering the flat? This would make far more sense: it would be a reasonable assumption that after having killed Margot, Swann would be in a hurry to get out and in his haste might forget to return the key; far better to have him put it back right after unlocking the door. This would also insure that if something went wrong and he had to flee in a hurry that the key would already be taken care of.
>>For that matter, why go through the trouble of stealing Margot's key? Why not just make a duplicate or two? Tony had had a year to do just that, at anytime and anywhere he chose. It would be practically untraceable, and in any case having a duplicate key to your own apartment is hardly some odd thing to do. Everyone has an extra key. Of course he'd still have to instruct Swann what to do, but he wouldn't have had to risk not being able to get Margot's key out of her handbag (all of which looked a bit obvious anyway).
>>What if Margot had gone out that evening? Tony couldn't guarantee she wouldn't. She would have missed her key, for starters. And if she didn't discover it was missing until after she came back, she'd have had no choice but to wait in the hall until Tony and Mark came back from the dinner...which would have led to all sorts of complications when Swann arrived to kill her.
>>Why wouldn't Tony at least check the key he took out of Swann's pocket? He had time to do so. At least then he would know it wasn't his and could put it back (wiping it for fingerprints, one hopes). As Inspector Hubbard points out, most men carry a latch key around with them. It's a pretty fundamental and foreseeable mistake to simply assume that his, Wendice's, key was the only one Swann happened to be carrying.
>>Much of Tony's plan depends on his calling the flat at precisely 11 PM. As we see, when he goes to do so, not only has his watch stopped -- making him late and calling attention to his phoning -- but the public phone is being used, forcing him to wait even longer. There's no way Tony could be certain of being able to get the phone at the exact hour he wants it -- it could have been a much longer delay. And as we see, Swann almost leaves when the call is late. Tony should have realized these potential problems and advised Swann that he would call him between, say, 11:00 and 11:15 -- a range of time rather than an exact minute.
>>His plan included calling the flat to wake Margot, then after Swann had finished to hang up and call his boss. It's a long shot, but a police investigation of the phone records could easily turn up the fact that a call had been made to the Wendice flat at the time of the murder.
And after Margot is convicted of murdering Swann, why does Wendice draw attention to himself by spending all those £1 notes? Why not exchange them for larger notes, reversing what he did in accumulating them over the previous year? He wouldn't even have to change all of them, but simply convert them here and there, even depositing a little money in the bank in small amounts at irregular intervals. And why do something that so plainly screams "guilt" as moving his bed into the living room? He's cold-blooded enough to plot to kill his wife yet his conscience won't allow him to sleep in their bedroom? A bit much, but from the point of view of getting away with the crime, it's an act that certainly calls attention to him.
ADDENDUM -- ABOUT THE KEY... We learn that Tony stole his wife's handbag six months earlier in order to see her letter from Mark. Later she tells Inspector Hubbard that when she got the bag back two weeks later the key was still in it. Tony then says that the thief could have copied the key and then replaced it, whereupon Hubbard says that no key was found on his body. So, two things: (1) If Margot's handbag was gone for two weeks why didn't she have a new key made? She had no way of knowing whether she'd ever get it back, so it's pretty ridiculous to believe she wouldn't have a new key made in that time...which would have eliminated the need for Tony to lift her key from the bag on the night of the attempted murder; he could just have left the new key under the stair carpet. (2) When Hubbard says no key was found on Swann's body, Wendice, if he was as clever as he thought he was, should have checked to see if the key was under the stair carpet. He took what he thought was his key out of Swann's pocket and put it in Margot's handbag. But he never checked to see if it was the right key or if Swann had another key on him. Hubbard's remark should have triggered a cautionary thought in Tony and led him to double-check the keys, since it should have struck him as odd that Swann had no key on him when he died...as it had struck Hubbard as odd.
And here's a (3): When Margot's bag was "stolen", with her name, address and key inside, why wouldn't they have had their locks changed and new keys made? That would be an obvious precaution. Of course the murder plot could have gone on as before, but this way Tony could have gotten extra keys made without arousing suspicion.END EDIT.
Yes, of course, the answer to all these questions is -- without them there wouldn't be a movie. Even so, Wendice's plot had a lot of obvious hazards he should have foreseen and made allowances for. Hardly "the perfect murder", even if the killing itself had come off.
Two other observations, nothing really to do with the film, but just to toss in....
When Tony goes to make his call at the club, there's a portrait of Queen Elizabeth II to the left of the phone booth. Today, 61 years later, the world has changed, telephone technology is vastly different, virtually everyone in the cast is dead...yet that same Queen still sits on the throne.
And as for the money Wendice has been saving up...well, Britain has long since gotten rid of one pound notes. Now it has only £1 coins. If this were a crime being plotted today, Tony, presumably, would not be paying Swann with an attaché case filled with 1000 heavy pound coins, or have been seen lugging pocketfuls of pound coins around London to pay off his debts. That would really be drawing attention to himself.