we are talking about an aircraft with no apparent unservicability
The AAIB could not say that the aircraft was definitely serviceable - I was there, in the House of Lords, when the investigator said it. No Evidence of unserviceability is No Evidence of anything I'm afraid.
As mentioned above - low level by your standards perhaps, but not by SH standards.
As a Flight Safety staff officer, would your verdict have been 'Unknown Cause' ? Only curious
Bear in mind that the original BoI (who had heard the evidence first hand) and the reviewing officers at HQ didn't blame the pilots. The idea of blaming the crew only came from the top at the end of the review process, to the astonishment of those below.