You really do pick 'em dont you...
After reading your thread about the guy that fitted your single disc, I would be thinking of using a reputable company to service your car from now on.
You didnt get an invoice cause you paid cash, therefore no tax was involved.
My advice to you is use a reputable garage, or even go to nissan from now on.
I dont want to seem like a grinch but thats 2 posts about what could be dodgey worksmanship. Your only putting yourself at risk, and it will only cost you more in the long run.
+1 - was it the same guy who did your brakes?
You have joined a forum of ZED enthusiasts who up and down the country can point you in the right direction for quality work and parts so USE IT and even if that means traveling a fair distance otherwise your ZED experience could well turn into a complete disaster.