Not sure if this helps but would the abbrev. MU stand for Maintenance Unit?
In the BCATP a Cat "A" Accident was defined as a total loss, a Cat "B" Accident was repairable by the manufacturer or a special repair depot and Cat "C" damage was repairable locally. Not sure if these were the same, expanded or differed in the U.K.
Those links are pure gold in terms of clarification and definition. I guess it sort of figures the RAF & RCAF had reversed alphabetical nomenclature for damage categories ... one of us drives on the wrong side of the road, too! Appreciate the expanded info and found the link material and contents excellent, too.