Focussing screen for the 645NII

I am the proud owner of a Pentax 645NII and have recently obtained some used Pentax SMC 645 A lenses and they work fine for me, but I now start to feel the need for a split-image / microprisms focussing screen just like I had on my Pentax ME. I did some Google searches and noticed that they do indeed sell these but I seem to have read somewhere that some of these, the once intended for the Pentax 645/645N won't fit on a 645NII, or so.

Can I install a focussing screen myself? Are there really differences between the focussing screens for the different Pentax 645 cameras? What else do I need to look out for? Where would you recommend buying these (preferably not eBay)?
I am in the UK, if that makes any difference.

Thanks!

I'm interested too, it's hard to find
Why it shouldn't fit on the NII?
I know this one http://www.tincheungcamera.com.hk/product/500

Hans can we organise a London film photo walk?

Sorry, I didn't see your comment until now. Anytime you want in a weekend

In the end I decided not to buy a focusing screen but rely completely on the focus indicator of the camera's viewfinder instead. I focus on the area or subject I want to get in focus until the indicator lights up in the viewfinder and then compose the shot; not great for street photography although that is how I shot this fast moving bride in Harajuku in Tokyo: http://flic.kr/p/cFvZNA