Black hearts in Battersea - Joan Aiken

"Simon paid off the driver and turned to follow Mrs Twite. But she seemed to have locked the door behind her and, as he rattled the latch unavailingly and then rapped the locker, something dark and suffocating was forced down over his head and a pair of hands gripped his throat."

Simon, coming to London to study painting with his old friend Dr Field, finds he has vanished without trace. Determined to discover what lies behind his disappearance, Simon is trapped in a fiendish plot. Why has his landlord got guns in his cellar and how does one deal with his irrepressible daughter, Dido . . .?