HONGKONG'S adopted son Mick Kinane recorded the biggest win of his success-laden career yesterday when he won the Epsom Derby on the Henry Cecil second string, Commander In Chief. The Derby was the seventh race on last night's final Happy Valley card of the season and Commander in Chief returned 6-1 in Hongkong and 15-2 in England. But there were plenty of punters ready to wade in locally on the Irish champion who turned his back on Sheik Mohammad to remain in Hongkong for six months of the year. And congratulatory messages were already beginning to flow back to Ireland and England from Hongkong last night as thousands of Valley viewers, plus a territory-wide television audience, saw the 33-year-old international ace score a devastating win on thePrince Khalid Abdullah horse. The Chief Executive of the Royal Hongkong Jockey Club, Major General Guy Watkins, said: ''It has all been a tremendous success with so many staying behind to watch the race. ''And we are all delighted that Mick Kinane has won it. When he decided to continue his career in Hongkong as well as Ireland it gave Hongkong racing a tremendous boost.'' Kinane, dubbed ''super sub'' when taking last-minute mounts in the past for Sheik Mohammad and then winning races like the King George VI and Queen Elizabeth Diamond Stakes, was coasting behind the pace set by former Hongkong dual champion jockey Philip Robinson on Bob's Return. Tenby, the odds-on favourite here and at Epsom, was outside the leader poised to make a run after rounding Tattenham Corner. But when Pat Eddery went to make his move, Tenby found nothing and interference which took place was later judged by an inquiry to have had no effect on the result. Tenby finished 10th. Kinane was coasting on the winner and could scarcely believe the ease with which he went to the front and then on to a victory as convincing as that of Generous two years ago and other notable performers such as Shergar and Troy in the past. Cecil said: ''I worried about everything coming too quickly, but the Prince decided to run him. It was his decision and he was responsible for the whole thing. It's a marvellous result. ''Commander In Chief should now go on improving, as he wasn't trained last year.'' Outsider Blue Judge (40-1 in Hongkong and 150-1 in England) finished second for veteran rider Bruce Raymond with Blues Traveller (45-1 here and 150-1) in England taking third place for Darryl Holland.