Indeed those advertising rules!
I have always found one mock to be enough. Three papers in one go. At this stage, if DC do not do well, don't tell them. Confidence is always key. I gave mock results to parents in a sealed envelope and told all my pupils that they had done well. After all, if they survived the time and a room with other children all panicking then they have done well.
I had a couple of pupils over the years with dodgy mock results, but these did not indicate a lack of success in the real exam. The point of a mock, especially this late, should only be to give the child an indication of what it is like to sit through exams for that length of time.