Lestrade and the Mirror of Murder

Book ten in the Inspector Lestrade series. Beyond the mountains of the moon ... 'Right, gentlemen. Recapping by numbers.' Superintendent Lestrade, in martinet mood, was driving his minions.
'Murder One. Four victims, Captain Orange, late of the merchant service and his three nieces, when the harness of their trap broke on a downhill gradient near Peter Tavy, Devon.'
'Clues?'
'A tall man seen near the Captain's horse shortly before the trap left. He could have cut the harness.'
'And?'
'A broken m...