Bookmakers are bracing themselves for an avalanche of bets from once-a-year punters when the £1 million Randox Grand National takes place on Saturday. The world famous Aintree race is the biggest betting race of the year when households from all over the country join regular punters in taking a financial interest in the outcome of the 4m2f race, which is being shown live by ITV . For Britain’s 6,000 betting shops it is the single biggest turnover day of the year and an equally busy occasion for