THE second bombshell to hit British racing in 72 hours landed last night with the announcement that champion jockey Peter Scudamore is to call it a day after 15 seasons of race-riding over jumps. Scudamore, the most successful jump jockey in racing's long history, has announced his retirement, and was scheduled to have his final three mounts at Ascot yesterday. But he says his plans to hang up his boots and saddle has nothing to do with the embarrassing shambles at Liverpool last Saturday, in which he rode Captain Dibble to complete a circuit of the course before pulling up. ''I am getting out while I am still at the top and still in one piece,'' Scudamore said. ''My retirement has absolutely nothing to do with the Grand National fiasco. I had been thinking about retiring for some time.'' Scudamore, 34, is giving up while still at the peak of his profession but admitted that the a younger, hungrier pack of weighing room rivals had emerged and were snapping at his heels. Richard Dunwoody, his long-time rival at the top of the jockeys' list, is now poised to pick up his first title, with the current champion abdicating with seven weeks of the season still remaining. Scudamore ends his career with eight jockeys' titles to his credit, and went into his final day with 1,676 winners to his credit, more than 300 more than John Francome, in whose shadow he lived a decade ago.