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.