Thomas, I'd assumed the 1000ft from the report's paragraph:
The recorded radar track started just after the helicopter departed the heliport. The
helicopter’s altitude as it approached the area of the accident was approximately
1,000 ft amsl and its average groundspeed was approximately 105 kt. The last radar
return reported an altitude of approximately 400 ft amsl, when corrected for ambient
air‑pressure.
I interpreted this as "a series of returns from 1000 feet towards the accident site, and then one final single return from 400 feet on the descent". That may be a misinterpretation, as this paragraph includes details from the whole flight. I think it does mean that there is an unambiguous radar track, even if I have it wrong from the AAIB text.
Why miss the red warnings? If there was a display problem, maybe the red central console warning lights just weren't there to see, and the two non-flight crew wouldn't have known where to look for their backup replacements. As to why the pilot didn't - well, that's the key question.