SOUTH African star Michael Roberts has become the first jockey to capitalise on Frankie Dettori's adventurous decision to break an eight-year association with Newmarket trainer Luca Cumani. In officially agreeing terms of his new contract with Sheik Mohammed - the offer both Steve Cauthen and Mick Kinane rejected - Roberts has been guaranteed the riding of all the leading owner's horses in Britain, including those trained by Cumani. Dettori, as confident as ever, has effectively burnt his biggest and most important ''bridge'' - Cumani's Bedford Lodge stables - in his quest to ride in Hongkong full time from September. He remains optimistic he will be in demand. Roberts agreed terms to take over as the number one jockey for Sheik Mohammed, as expected, in a two-hour meeting with Anthony Stroud, the Sheik's racing manager, in Cape Town earlier this week. But the reigning champion will be giving his rivals an earlystart as he mounts his first title defence. As part of the Sheik's pre-season preparations, Roberts will be required in Dubai for strategy talks when the Flat season (turf) commences and thus misses the first two days. ''But I'm hoping to be back in Britain for the Lincoln Handicap on the Saturday (March 27),'' said Roberts from South Africa. ''Really, though, it all depends on what Sheik Mohammed wants me to do.'' Also included in the deal is first call on all Sheik Mohammed's horses trained in Ireland and France, with the one exception being those prepared by Andre Fabre, who will employ his own stable jockey, Thierry Jarnet. Roberts said he was pleased at his contract and the possibilities. ''I got quite a shock when I looked at the number of three-year-olds I will be able to ride. There are more than 300 there, and that doesn't include the ones trained by Andre Fabre. It is mind-boggling.'' Roberts is booked to return to Newmarket on March 14, earlier than usual, and will be taking a four-day tour of the Sheik's French horses. He has been asked to look in at Fabre's yard while there. He can be called on to ride with Francois Boutin, John Hammond, Criquette Head and at other yards in which the owner has horses stabled in France.