LONDON – Booming art prices have produced plenty of “treasures in the attic” of late, and this week could see another when a painting first valued at 300 pounds is set to fetch up to three million after Bonhams discovered it was by Velazquez. The portrait of an unknown gentleman goes under the hammer on Wednesday, as London hosts a series of old master and British art auctions featuring works worth tens of millions of pounds. While the newly discovered Velazquez is not the most valuable lot on offer at Christie’s, Sotheby’s and Bonhams, its story is arguably the most arresting. The painting of a balding man in black tunic and white “golilla” collar was part of a small collection of works attributed to 19th century British painter Matthew Shepperson which was consigned for sale at Bonhams in Oxford.