A group of hop-pickers on their way from work approaches (Larklake) Bridge in a horse-drawn vehicle. The horses shy; the vehicle plunges over the bridge into the River Medway with great loss of life
[On October 20, 1853,] A horse-drawn brake carrying a party of hop-pickers plunged over Hartlake Bridge into the River Medway. Thirty people, including four children, were drowned. The dead included Travellers, Irish, and English.
[MacColl and Seeger write,] "In spite of being very well known among Kent and Surrey Travellers, the song does not appear to have been printed at any time." - PJS
Regarding the date of the event, Hall, notes to Voice08, re "Hartlake Bridge" cites Mike Yates as source for an October 1858 date. Yates, Musical Traditions site _Voice of the People suite_ "Notes - Volume 8" - 1.3.03 also has the date as October, 1858. - BS
It appears this is a nmisreading. I found a reference to the accident in the October 29, 1853 edition of the _London Illustrated News_. It claims 32 people were killed. As of this writing, a citation may be found at http://www.iln.org.uk/iln_years/year/1853.htm. - RBW