Bron, the Tinkers' album Oldham's Burning Sands has a note about the song which is pretty much in line with what I learnt at school: "...some 80,000 people gathered to hear the well-known Radical, 'Orator' Hunt. The yeomanry charged into the crowd, laying about with their sabres. In a few minutes, eleven people were killed and some 400 injured."

I have always thought the tragedy might partly have been explained by both sides underestimating the effect of charging horses in a large crowd. Anyone who was at Grosvenor Square in '68 will know what I mean.

It is unlikely that the news in those days would have been massaged to the benefit of the rioters. The occasion deserved to make a massive impact in its own right. Percy Bysshe Shelley was inspired by the incident to write an angry ballad, the Masque of Anarchy, which is well worth a read. Its lines include:
I met Murder on the way—
He had a mask like Castlereagh...

His [Lord Ermine's] big tears, for he wept well,
Turned to millstones as they fell;
And the little children who
Round his feet played to and fro,
Thinking every tear a gem,
Had their brains knocked out by them....

Stand ye calm and resolute,
Like a forest close and mute,
With folded arms, and looks which are
Weapons of an unvanquished war.

And let Panic, who outspeeds
The career of armed steeds,
Pass a disregarded shade,
Through your phalanx undismayed...

And perhaps most famously:

Rise like lions after slumber
In unvanquishable number!
Shake your chains to earth like dew
Which in sleep had fallen on you:
Ye are many—they are few!