F80 Focusing Issue

[Dr_Q]

After keeping my F80 cupboard bound for some years I took it out and had a play with it, worryingly nothing was properly in focus. I managed to fix this by adjusting the dioptre on the viewfinder, I assumed it was actually a problem with my eyes and not the camera. This was 2 years ago.

Yesterday after being in the cupboard for a while longer I had a quick play and found that if anything the problem was no worse, even with the dioptre adjustment. I decided (probably stupidly) that this was probably a focusing screen issue so I reseated it to see if there was any improvement, sadly there wasn't. In my desperation I then tried to have the focusing screen in any orientation possible, none of these fixed this particular issue.

I don't know what could have happened, there is an outside chance the the body was knocked slightly but I can't recall it falling or taking any serious damage. I'm not the most practical person when it comes to fixing things but I'm more than willing to give it a go. I had a look at the prices for getting it done "professionally" and it would cost the same as an overpriced second hand body!

Can anyone think of a cause and even better, a fix for this?