Rostropovich stakes Derby claim with Dee Stakes victory at Chester

• Aidan O’Brien yet to decide whether son of Frankel races at Epsom• O’Brien and Ryan Moore seal Chester double after Idaho winA bookmaker was offering just 1-5 on Aidan O’Brien winning a record-equalling seventh Derby next month after his colt Rostropovich took the Dee Stakes here on Thursday and, though there are plenty of Classic trials to come over the next two weeks, the implication that O’Brien has around an 80% chance of saddling this year’s Epsom w