Olympus E-520 blurred viewfinder (1 Viewer)

Having a problem with a blurred viewfinder on an Olympus E-510, it only became apparent when I started to try to manual focus. It really became apparent when I got the Olympus magnifier attachment for my viewfinder
Checked the camera again this morning and the blurriness runs up and down the viewfinder, anything to the left or right of the viewfinder aiming circle is clear, but anything within the viewfinder aiming circle or above and below it is blurry. It is less blurry on the right side of the aiming circle but gets worse as you move across to the left, but anything outside the aiming circle on the left or right is fine.
This became apparent when I first started to try and manual focus, couldn't figure out why my images were always out of focus, thought it was my eyes.The blurred sighting ring became apparent yesterday when we were both testing the camera, and I was trying a Canon XS to see how well I could manual focus it. My wife noticed it too.
The camera autofocuses just fine although the blurriness is still present on the viewfinder the image captured is still in focus. I might just try manual focusing with the left side of the viewfinder and see how that works.
I've cleaned everything that is accessible, but the nature of the blurriness leads me to think it is a defect in the viewfinder. If it were condensation I would think that the blurriness would change a little from day to day and you would at some point see a mist like covering or droplets, but I don't just a a slight blurriness in the areas I have described.
I have tried 2 different lenses and the blurriness is still present.
Anyone got any advice or ideas.

I have to apologize for the title of the post it is actually an E-510.

What happens when you adjust the diopter control on the viewfinder, can you get the centre sharp and defocus the edges, or does it stay blurred whatever you do? As the 510 focussing screen is fixed it does seem that something needs a professional assessment and fix.
From memory the E510 uses mirrors rather than a solid glass pentaprism to erect the image and its possible that a misalignment where two meet at the centre might cause the effect you see
The 510 viewfinder always seemed a lot clearer than that on the original 500, but even with the magnifying eyepiece I never felt either were easy to focus manually, but then I still have my film OMs which spoil you for anything else!

Thanks for the reply.
Diopter is adjusted correctly, anything outside the sighting circle is in focus.
Posted on DPreview and got several good replies.
Inspected mirror and bottom of viewfinder with a flashlight. While moving flashlight beam across the bottom of the viewfinder I discovered that the left side of the aiming circle is comprised of 3 lines rather than one line as on the right side of the circle. It appears to be a manufacturing defect. The left side of the circle is where the blurriness is the worst.

Problem resolved.
Had also posted on dpreview.
It turns out the retaining clip for the focus screen was loose, it actually fell out when I touched it. The focus screen also fell out. Used instructions from katzeye to reinstall focus screen and retaining clip. Bluriness gone, manual focusing now possible.