In the play Spider's Web, Oliver Costello was the boyfriend of Miranda Hailsham-Brown. He also supplied her with drugs. Mr Costello lived at 27 Morgan Mansions, SW3.
He arrived at Copplestone Court wanting to see Mrs Brown. When he was greeted by Clarissa Hailsham-Brown he seemed surprised, but told her that Miranda wished to have Pippa live with her. Pippa comes into the drawing room, and becomes very upset when she sees Mr Costello. Clarissa and Mildred Peake throw him out from the house. Later that evening Clarissa stumbles over his dead body.
Even later Clarissa figured out that the Mrs Brown that Oliver had wanted to see wasn't her, but Miss Peake.