Bill Chorley's OTU volume in his wonderful Bomber Command Losses series says that HF600 took off from Turweston at 2020 hrs for a Bullseye Exercise (a navigational detail simulating a bombing raid on a nominated "target") Returning to base in the early hours of the following morning,control was lost while orbiting a beacon and the aircraft dived into the ground at 0114 hrs. It would seem that it was probably vertigo and pilot inexperience rather than an aircraft fault that caused this accident. However, OTU Wellingtons were often a bit past it. What the Americans called - officially - war-weary. The OTU losses book makes grim reading.
The Accident Investigation Branch only analysed certain accidents when called upon by the RAF. Some of these reports are obtainable from The National Archives at Kew. More details on this link.
RAF-lincolnshire.info :: historical sources