AAIB findings
Both pilots were qualified and licensed to operate the aircraft. By having a mentor pilot on-board the PIC showed himself to be safety aware. The report sees no need to mention the legalities of the operation of the flight which means that these factors had no relevance to the accident. By making safety rec 2010-15 it seems that the AAIB believe the aircraft manufacturer should do more to make pilots aware of the possibility of aborting engine restarts unintentionally.
As with all air accidents, this report shows that it is a sequence of unforeseen events that caused the loss. Had these events occurred uniquely or in isolation they would not have caused the aircraft to crash. Some may say that it was bad luck.