I didn't know about the Odiham Chinook probably because it wasn't fatal.
Mannheim was a result of the forward pinion bearing seizing which led to the transmission failing. Rumour has it that Elfinsafety stopped the overhaul team used 'dangerous' 3,000 p.s.i. to clear the oilways of the walnut grit used to clean them. Lower pressure didn't work as well so it eventually blocked it.
The Sumburgh on was where a solid gear was replaced with a lined one that could be overhauled more cheaply. Unfortunately it wasn't trialled over the North Sea so salt laden air got in and corroded the joint leading to a failure of the forward gearbox.
You can make helicopters 100% safe but then you get slammed by a bus on the way to work.