Ouch, that's not cheap at all is it?
Can you not find a German Zed forum, ask for the details of a local garage in Koblenz, then arrange for the car to be taken there to be fixed for less? Or (and I like this idea), find out the cost of a master cylinder change at Nissan dealer in the UK, then tell the Germans you're happy for them to fix it but will only pay the price that you've been quoted in the UK? Can copy Nissan GB and Nissan Germany into the email chain too, just for lols.
The trouble is, how much ballache do you want to go through and then how much more are you going to save by faffing around? Oh, and get the slave cylinder done at the same time, labour is the same so you should only pay for the part really.