Unsure as to what you have or haven't done...
I'd try speaking with the General Manager at Bishops and highlight this case to make him aware. The Service Manager could have omitted to telling him about this incident. . .it happens.
If that doesn't work, I'd go up to the next step in the ladder i.e the top man for the Bishop's franchise.
If that doesn't work then it's off to Nissan GB, telling them that dealer has been unsupportive. . .etc.
If that doesn't work then I do as some of the other guys have suggested.
Finally, if that doesn't work whatever you do, don't watch "Falling Down" with Michael Douglas ...it could get messy
Good luck
Ross