Unfortunately, I do not know any source of Nital available for individual use in Europe.
I used to get my Nital from THE ETCHANT STORE in the US.
Several years ago, I could order it and have it shipped directly to the Netherlands.
A few years ago, they changed the policy and shipped only to the US and Canada, so I had to ask a fellow forum member from the US to be kind enough to re-ship it to me.
Now, they do not serve private individuals anymore and I was not able to order from them.
Now, I switched to Ferric Chloride, but I am not happy with the change. I am still looking for a source of Nital in Europe.
Meanwhile, I either use Ferric (which I hate), or send the blades to a friend of mine here in the Netherlands who does the etching with a nitric based solution (similar to Nital). He is also a very skilled with various types of repairs.
At the time I was able to order from the US, I would combine equal parts of Nital 3% with Nital 5% to get Nital 4%, which I found to work better.
How did I use the Nital for etching?! See the link below (with the observation that I got best etching results with a surface polished to about 2000 grit):
http://www.vikingsword.com/vb/showthread.php?t=21732
PS: I am not very familiar with the metallurgy of wootz so I do not know about differential tempering of wootz.
However, I certainly did not hear about the use of differential tempering on wootz blades and I suspect it might adversely affect the wootz structure/watering pattern.