DISCOUNT retailer B&M was valued at more than £2.7 billion yesterday in a stock market flotation that will deliver a £1bn windfall for its backers. The chain, set up in 1978 and is now among the UK's biggest retailers with 373 stores and 16,000 staff, saw its shares rise in conditional dealings from an opening price of 270p to as high as 289p at one stage. Manchester-based brothers Simon, Bobby and Robin Arora, who bought the chain in 2004, will get another payday from the float, having already received an estimated £900 million when they sold a controlling stake to a private equity firm in December 2012.