BIG-STRIDING chestnut Armiger, the third member of powerful owner Khalid Abdullah's Epsom Derby squad, faces a relatively easy task in today's Prix Lupin at Longchamp, his final prep race before next month's Classics. The Henry Cecil-trained colt, supplemented at a cost of 50,000 francs, has only four rivals, which he should see off without any great worry. ''The main reason we are going to France, instead of the Predominate Stakes at Goodwood (on Tuesday), is that there is a much better chance of getting the ground that he really likes,'' explained Cecil. ''He likes a bit of cut underfoot, and what we don't want is to show up and find lightning fast going,'' he added. But the trainer should not lose any sleep on that score following the persistent rain in Paris over the past seven days. Although the Prix du Jockey-Club at Chantilly is generally accepted by both bookmakers and ante-post punters as Armiger's likely Derby target - leaving the Derby at Epsom on June 2 to his stablemate, the 4-6 favourite Tenby - the matter is far from settled. Prince Khalid will make the final decision, and the going at Epsom is likely to be the deciding factor. If by some remote chance, the heavens opened, providing ground similar to that served up in Teenoso's year (1983), I believe Armiger, now 8-1, would be saddled up in the world's most famous Flat race. Of course, everything would change immediately if Tenby were injured or suffered some set-back, as Chester Vase winner Armiger would then assume the pole position, irrespective of the going. Tenby consolidated his position at the top of the Derby market with a tidy, all-the-way victory in the Tote Dante Stakes at York on Wednesday. There could be no faulting this trial, although the win was not spectacular. Why then was Tenby slashed to 4-5 from 6-4, one might well ask? The basic reason is lack of opposition. The assumption is that Prince Khalid will send only one or two runners to Epsom, which means there is not an awful lot left. Experts were warning of the possibility of a field below 10 runners - I'm sure that Hongkong betting officials must be praying for 14 - which would set a modern-day numerical low. The last time there were fewer that 10 was back in the 1907 Derby, won by Orby. Tenby, incidentally, is a small colt, but one who has a lovely temperament and a relaxed nature. He can also fight out a finish, as he displayed when taking the Newmarket Stakes on his way to the Dante. ''He's the Derby colt for me,'' declared Pat Eddery, who will be chasing his fourth Epsom triumph. ''He's so relaxed you can ride him anywhere in a race. He's won from the front, and from behind. ''He's also won on all types of ground, from fast to heavy, and has proved he can act on tracks as different as Goodwood, Longchamp, York and Newmarket. How can you go past him?'' It was very hard to disagree with Eddery's summing up, even at this earlystage. The other significant occurrence at York during the week was the performance of another member of the Abdullah Derby squad, Commander In Chief, a half-brother by Dancing Brave to outstanding miler Warning and the very capable Deploy. Commander In Chief was being earmarked for Epsom as the Abdullah second string, behind Tenby, but those plans seem certain to be revised after his battling win in the Glasgow Conditions Stakes. The colt scraped home by a neck from Clive Brittain's Italian Derby aspirant Needle Gun, and Eddery declared: ''Commander In Chief seems too immature at this stage for Epsom. He still has a lot of learning ahead of him. ''I would think the King Edward VII Stakes at Royal Ascot would be best for him at this stage,'' the jockey added. In my view, Grant Pritchard-Gordon, Abdullah's racing manager, came up with the best line: ''Any owner would be happy to have Commander In Chief run for them in the Derby. But Prince Khalid is in the happy position of being able to choose from three or four.'' It certainly is an incredibly strong hand being held by the Saudi prince, arguably the strongest since the previous Aga Khan back in the 1930s. Before I close this week, one important footnote concerning Frankie Dettori, the young riding talent whose named has been linked with a cocaine arrest in London in April. Dettori, who was not charged, rides this weekend for The Queen - he partners Enharmonic in the Badener Meile in Germany.