The arrest of a 25-year-old local man follows the arrest of a 52-year-old man on Monday who remains in custody.
The latest development in the investigation comes as details of a post-mortem examination became public confirming that Mercer, whose body was found outside her home in West Molesey, had died from “multiple blows to the head”.
The dealer who worked at JH Bourdon-Smith in Mayfair and was a respected member of the antique silver trade, was found lying dead in her front garden by a neighbour at around 8am on Monday.
According to press reports, police found her body with injuries so severe that they “thought she had been shot”.
Tributes have been paid to Mercer by Victoria Borwick, the Conservative MP for Kensington and President of dealers’ association BADA, as well as by members of the trade.
Outside of her professional life, Mercer was a keen rower and charity fundraiser. The Walton Rowing Club, where she was a member, posted a message on its website: “The club has been shocked to learn of the untimely death of one of its members, Robyn Mercer. We offer our deepest sympathy and condolences to her family.”
