I checked a few of your proportions and noticed some areas that were off.  Here, Ive just drawn a bunch of lines all over your drawing


(whenever I refer to left or right side, Im talking about our left/right, not john cleese's, I think thats who it is)

First thing I noticed is that when you divide the face up into thirds, the top of the middle third is right at the bottom of his eyebrows, but you have it in the middle of his eyes.

I think the mouth might be a little too far to the right, atleast the right side of it seems to go out too far.

the pink lines just show the angle of the face, I think youre curving the angle in a little too high up.

with the blue lines I tried to show that I think the mouth needs to angle up slighly more on the left.  Also, you have the right side of his upper lip above the left side which looks a little off, try just making it even with the left side.

another thing I just noticed is you have the right eyebrow slightly higher than the left, but it should be the other way around ( put a horizontal line on the top of the eyebrows on the ref to check).

Once you get the proportions right, the eyes and mouth will help alot to make it look like him, its a pretty subtle expression, so its a little tricky.

Here, I think the nose is angled an unusual amount towards the viewer. In the photograph, Mr. Cleese's nose is pointing in the direction he's looking. Try bringing the nose around so tat it's angled away from the viewer more. Another thing about this expression that's very, very important is the line of Cleese's shoulders. He is an excellent physical comedian, and his facial expressions really shine when put into context by his body. The inclusion of his neck and body is a little extra work, but it's part of his expression as much as anything (in my opinion, at least).