http://www.erebus.co.nz/Portals/4/Do...ot%20error.pdf
The radar issue is dealt with at pages 120 to 127.
Whatever Mahon might have to say on the subject, and he says quite a lot, the radar had no relevance, because the captain descended into the danger zone on the basis that he was flying visually. The word "visually" does not refer to what one can see on the radar screen inside the cockpit. Instead, it refers to what one can see out the window, outside the cockpit.
As a matter of fact, the conditions were not suitable for a visual descent, because white cloud above would merge with the white ice below, making one indistinguishable from the other. Did the captain know that before he descended? Most definitely: He expressly said so 10 minutes before he did it.