I remember reading this in an anthology around 2000. Setting was London, and the main character was an older gentleman with a sword cane who was a magician. He could see imps darting around electrical devices, and there was a young woman with a captive imp she used to scare off men. there was a plot involving gangsters and an old rock and roller who had been purged of his demons to lay a trap for the investigator. in the end there was something in the river, a revnant, that nearly killed the girl by eating her imp, the older gentleman cut its bond with his sword cane.
It starts with the protagonist following an apparent cutpurse through the London fog an night, but the cutpurse turns out to be the young woman with the imp. The imp is eaten by a revenant that then attacks the woman but is driven off by the protagonist (with his sword cane).
I stepped up and with a short stroke severed the umbilicus that linked the cutpurse to the revenant. It slurped up the cut end like a length of spaghetti and turned towards me. Whatever human qualities it might once have possessed had worn away long ago, leaving little more than a blind, bottomless appetite. For a moment, as I menaced it with my blade and tried and failed to dismiss it, it stood within my head, and I was jolted by a sudden, freezing headache. It reared back and stared at me; then its tiny, wide-mouthed head, like that of some species of deep-sea fish all maw and stomach, whipped sideways and snapped at the cutpurse.
“Your familiar,” I said, countering the revenant’s quick, sinuous moves with my blade. “It wants what’s left of your familiar.”
The revenant lunged at the cutpurse with jaws that were now as wide as a shark’s. I caught her wrist, broke off the knotted remnant of the imp’s tail, and threw it at the monster, which snapped up the trifle and withdrew as swiftly as thought. I ran to the crown of the bridge, looked over the parapet and saw something faintly luminous and very long pour into the canal’s black water
The girl is Miranda and the protagonist describes himself as:
“I am Mr Carlyle. I have the honour of being a consultant in the matter of the dead.”
The old rock star is Rainer Sue.
The gangster, Donny Halliwell, is after an eldritch tome owned by Carlyle:
“In a way. The Stenographia is the masterwork of a monk and magician who called himself Trithemius, and contains codes and conjurations and various prayers which its author claimed could cause angels to act on behalf of those deploying them. My copy is not of the much corrupted edition that was published long after Trithemius’s death, in 1676, but one of only five volumes printed in 1504, the year before he was summoned before Maximillian I and interrogated on matters of faith.
The mastermind behind the gangsters is Cagliostro. He wants the book to bind the revenant from the river into his service (the same revenant that attached Miranda at in the open paragraphs), and he plans to sacrifice Miranda to the revenant as part of the procedure.