The aircraft involved was planned to be in Germany for his visit but the aircaft were changed at the last moment.
That would be a very long haul for a Westland Whirlwind HCC12.
Yes it was a machining flaw in the main rotor drive shaft. After the accident only twin engine helicopters were used by the Royal Flight. (They still had a single main rotor shaft though...)
The surviving Whirlwind HCC12 , XR 486, was transferred to 32 Squadron at Northolt. Externally you could differentiate between a HCC12 and the HCC10s by the double cabin windows on a 12. 32's HCC 10s were HAR10s modified for VIP transport by the addition of cabin linings, airline seats and red carpets. (Oh yes, and nutplates for mounting a star plate according to the rank of the occupant)