I'm so sorry that you have been treated this way, Susan. Believe it or not, there are plenty of good tradesmen out there, you have been very unlucky.
A decent builder would take note of your concerns and put it right without question. I don't think you're going to get anywhere with this lot though.
As has been said, 5mm is not a great amount, but nevertheless, it's not good. 5mm in one spot, but what about the rest of it?
I'd love to run a straight-edge across the whole floor. If they've got this spot wrong, what about the rest?
Whatever floor covering you're going for, a good tradesman will level it for you before laying it. I would definitely go for a floor levelling compound to put a lousy job right.