THE Derby, whether it's in Hongkong, Epsom or India, holds a fascination for anyone involved in racing and, locally, none is more enamoured by the Blue Riband than former champion trainer Brian Kan Ping-chee. Every time he goes shopping for horses overseas he looks out potential Derby contenders and he has come up with the goods on four occasions, including last year when Sound Print became the first horse to complete the Classic Trial and Derby double. Today he is represented by yet another Irish import, Happy Guy, who also won the Classic Trial last month for local ace Tony Cruz. And suddenly, with yesterday's dramatic withdrawal of favourite Deerfield, Happy Guy comes even more firmly into Derby calculations. A win for Cruz would admirably illustrate the ups and downs of the racing game. He took a tumble from low grade Rare Encounter at Happy Valley on Wednesday night, forfeiting five rides including a winning one, and ended up with a stiff shoulder and sprained thumb. But he made sure yesterday that he would be fit for Derby Day duty by working out a string of horses and said: ''I'm feeling good and I'm ready for it. I am sure he can run a good race and who knows, with a bit of luck, we might win it.'' Kan's love affair with the Hongkong Derby is well known and he has certainly put plenty of effort into getting Happy Guy ready for the race. Earlier, it looked as if Right Way would be Kan's main hope but he has been overshadowed lately by his stablemate. Apart from finishing behind Happy Guy in the Classic Trial, we was completely trounced in a significant piece of work that included Sound Print, so Happy Guy looks to be smack on target - if he runs out the trip. Kan says: ''He has form in Ireland over 1,700 and 1,800 metres and he has done very well since the Classic Trial. Last year I was sure Sound Print would win and earlier I thought Right Way would do it. ''But I am very pleased with Happy Guy and he could win.'' If he is slightly more reserved than in other years, it is understandable as there is now a definitely more open look about the Classic. Frankly, this is a Derby which would not seem to take a lot of winning. That was admirably summed up by champion trainer John Moore who had pencilled in a start for exciting Fastabob before the Australian import bled. ''I think a lot of them will have trouble staying the trip and it is the sort of race that a brilliant type like Fastabob, who is probably best at a mile, could win. ''The Classic Trial, I think, showed that there was not a lot of depth to the field,'' Moore said. It is difficult to make much of a case for horses who finished out of the frame in the Classic Trial - a point made by Irish champion Mick Kinane, who partners Magnified. ''The Classic Trial was a good guide to it last year and you would have to say that Happy Guy would have us covered if he does stay the trip. ''My fellow has come on a bit and so I'm sure have some others, but they have a lot to make up on Happy Guy. And it's also logical to assume that happy Guy has come on a bit,'' he said. Patrick Biancone's decision to start Helene Star has definitely added considerable lustre to the race. The horse has been in brilliant form but was being aimed for the International Bowl. But, as Biancone points out, the horse has done plenty of long, steady work over the past few weeks as the trainer attempts to get him more relaxed and settled. And there is another very important point when assessing Helene Star's chances. If this was a handicap, rather than a set-weights affair, Helene Star would be giving virtually the entire field lumps of weight. He is undeniably a top quality galloper and it may well be that Biancone, master trainer that he is, may yet win the Derby - with a horse not guaranteed to be in the line-up until the 11th hour. Those looking for an outsider in the field would have to consider a horse who did not run in the Classic Trial and therefore has some hope of causing an upset. Fitting the picture is the David Hill-trained Red Ruffian who has been coming home very strongly and stylishly at the end of his recent races. The step up in distance is dramatic but his breeding would suggest that, marginally, he could see out the trip. Progeny of his sire, Red Sunset, are probably best at just over a mile but on the dam's side, they have won over this trip. And what of Northern Fire Ball? The horse won quite brilliantly a week ago, coming from near last to simply cut down his field in the final event. That was course and distance and Darren Beadman was promptly engaged to ride him. Late last season Northern Fire Ball was an early tip for the Classic following a couple of superb griffin runs but he had been off the scene for three months before re-appearing and winning so well last week. But he now backs up just seven days later so it cannot be termed an ideal preparation for the race. And backing up just four days after defeat in a midweek event at Happy Valley is Perkyman. The task would seem to be beyond him and it also looks too tough for Al Bundy, Crown Commander, Optic Empire and Silver Arrow.