Problem with artefact/smudge on view finder

Over the last week my D3100 has developed a small circular artefact or smudge (it is transparent), on the viewfinder. The artefact is not in the lens which I have cleaned, and it is not on the sensor or the mirror as it is not appearing on the pics I take.
The main issue is that the artefact is affecting the ability for the camera to focus using the central focus spot! which it wont accurately do anymore.
With the lens off and looking into the body of the camera I suspect the mark is on the inside of the viewfinder glass which I understand might be removable. Can someone more knowledgable confirm or otherwise that the glass is indeed removable and therefore can be cleaned on both sides.

Cheers

​Paul

If it's on the last plane of glass that you look through (by that I mean the glass closest to your eye when you look thru the VF) then yes, it does come off by twisting the round ring that holds it in. I've removed mine to put in an extended rubber eye piece and while it was out I gave it a quick clean.

Dave,

Unfortunately I think its on the back of the pane you see if you take the lens off and look into the body from where the lens should be.

Ah, I see. I take it that blowing air (NOT from a can but from a blower) does not help, huh?

Unfortunately not.
Just found a link somewhere else showing how to remove this pane on a D5000 so I guess its held in the same way.
​

I don't see how something on the viewfinder would affect focus. I don't doubt that you have a focus problem, but it might be a coincidence.
There is a picture of the light path on this page. I think most of our dslr cameras work like this.
My Photography

Thanks for the comments guys.

I think I must have just been having a funny few minutes with the auto-focus issues. Camera seems to be auto-focusing fine now, but still got the circle around the centre focus point.

Having fun with my new 55-300 lens so will try and get used to the smudge and plod on.