HC,
The existing HUMS tools was quite capable of detecting the fault before it became critical, but unfortunately the tool was not used optimally and so the fault went undetected until the shaft broke.
Maybe on the second 225 ditching, but the Bond G-REDW AAIB report quite clearly says that Bond picked up a trend on the day before the ditching, and again after first flight on the day itself, carried out the AMM actions in both cases, and put the ac on close monitoring, all completely in accordance with the AMM. Could they have looked more closely, maybe, I'm sure you'll tell us Bristow would have done, but to say they did not detect it because they were not using HUMS properly is untrue.
Page 7 if you're interested:
http://www.aaib.gov.uk/cms_resources...2%20G-REDW.pdf