What do you think I should do?
When you look through the viewfinder, the focus screen is in focus. The mirror is not. If the dirt is in focus, it is likely to be on the focus screen.
I shoot Canon but I expect tht it is the same with Nikon:
- The focus screen is a user changeable component.
- They cost about 25 quid.
- They come with a little tool for changing them with and a box for storage.
I recommend changing the plain focus screen for a grid one anyway, it is so much easier to get the horizon straight...
Using the kit, you can take the screen out and point a rocket blower at it.
So, yes, I think you can do it yourself.
On the other hand, having done it once, I will be taking it to a pro next time. It's a horrible, fiddly job. You are dealing with delicate, fiddling things and need to apply enough pressure to make the screen snap-fit. I could recommend a place near Victoria station, London, but not in Northern Ireland.
Andrew