James, would be useful if you posted the photograph(s)? of the crash scene as this would eliminate some aspects.
We're working with precious little at the moment and anything related to the crash would help.
N.B. Reading the Lanc landing drill, flaps 20 were were selected before turning on to the downwind leg, presuming that this is the same for all heavies which would make sense, this would reduce the possibility of assymetic flap being a cause.
Stirling's had a lot in common with the Sunderland, the wing is a shortened Sunderland wing made thicker to compensate, this gave good handling albeit at the expense of lift and ultimately ceiling.