The buyer, one of 16 phone bidders, was an international gallery.
Although cautiously catalogued as a late 18th century Grand Tour piece, the bust was deemed to be ancient with later additions.
The youthful, clean-shaven subject is probably the Macedonian general Alexander (356-323 BC) depicted in heroic manner wearing the lion cowl of Hercules. Excluding the later socle base, it measured 15in (38cm) high.
It came with a good provenance. It had once been part of the furnishings at Sutton Place, the Italianate Tudor mansion in Guildford built by Sir Richard Weston, courtier to Henry VIII.
In modern times, the estate has had a series of wealthy owners, including the Duke of Sutherland (1919), John Paul Getty (1959), Stanley Seeger (1980) and Frederick Koch (1990).
The price, way above the £600-1000 estimate, represents a new house record for the Cheshire firm. The hammer price of £320,000 will jump to around £385,000 with the 20% buyer’s premium added.