Ooooh, that's one I sometimes travel on. Normally empties out quickly north of Hitchin.
The 1840 is just over 10 minutes behind by the time it reaches BIW (often less in practice), so little wonder!
I hate the 1813, but that's only because I end up getting it only when I've missed the 1743 (the 1813 HIT-BIW), usually by a matter of a few seconds!