In 2001 he also made his first-class debut, playing a single game against Sussex and making an immediate impact by taking 5–97 in the first innings, his maiden victim at this level being the opposing captain, Chris Adams.
He remained with Gloucestershire for three more seasons, making 92 (again from number nine) in the Championship against Warwickshire in August 2004, but first-team opportunities grew increasingly thin on the ground, until by 2005 he could manage only a single appearance.
In October of that year Sillence signed a two-year contract with Worcestershire, and bided his time until then by spending the winter in Australia, playing for Casey-South Melbourne in Victorian Premier Cricket.
Sillence returned to the first team immediately thereafter, managing to establish himself as a first-choice player, and in June recorded career-best figures of 7–96 in the first innings against Somerset.
's professional in the North Staffs and South Cheshire League Premier Division 'A', substituting for Bruce Martin, a New Zealander who proved unable to appear because of visa regulations.