Complex sentence: When Jane reached 21, in 1790, Perceval's career was still not prospering, and Sir Thomas still opposed the marriage, so the couple eloped and married by special licence in East Grinstead.
Split into simpler sentences: When Jane reached 21 in 1790 Perceval's career was still not prospering, and Sir Thomas still opposed the marriage. So the couple eloped and married by special licence in East Grinstead.